Why is my car key fob not working?
The most common cause is a flat or weak battery inside the key fob. A battery that is beginning to fail often causes reduced operating range or intermittent locking before the remote stops responding completely. Replacing the battery with the correct type and polarity is therefore one of the best first checks when one key has developed a remote-control problem.
Do not assume every non-working remote needs a new key, however. Worn buttons, damaged battery contacts, water damage, lost synchronisation and internal circuit faults can all affect an individual fob. If the key unlocks the vehicle but the dashboard reports that the key is not detected, the problem may instead involve key recognition or the immobiliser side of the system.
The spare key is one of the most useful diagnostic tools available. If the spare works normally, attention can stay with the faulty key. If both keys fail, especially at the same time, it becomes less likely that two separate key batteries or circuit boards failed together. At that point, check the vehicle battery, fuses, remote receiver, central locking system, body electronics and immobiliser before buying replacement keys.
One key doesn't work
Start with that key. Check its battery, polarity, battery contacts, buttons, case condition and whether the spare key works normally.
Both keys don't work
Move the diagnosis towards the vehicle. Check battery voltage, locking-system operation, relevant fuses, receiver operation and key-recognition faults.
Unlocks but won't start
Remote locking and engine-start authorisation are not always the same function. The car may receive the locking command but fail to recognise the key for starting.
Diagnose the pattern before buying parts. Compare the main key with the spare, note whether lock, unlock and starting functions behave differently, and check whether the dashboard displays a key or immobiliser message. Those simple observations often tell you whether to investigate the key itself or the vehicle.
If only one remote has poor range or has stopped operating, fit the correct fob battery carefully and test it again. If both keys have stopped working or other electrical symptoms appeared at the same time, check the vehicle side before spending money on replacement keys.
What is your car key fob actually doing?
Before replacing a battery, programming a key or buying a new fob, identify exactly which functions have failed. A remote key can perform several different jobs, and those jobs do not always depend on the same part of the system.
Remote locking, the mechanical key blade and engine-start authorisation can behave differently. For example, a key may unlock the doors normally but still produce a key not detected message when you try to start the car. Equally, a dead remote-control battery does not automatically mean the immobiliser transponder inside the key has failed.
Remote does nothing
If pressing lock or unlock produces no response at all, start with the fob battery, battery orientation, internal contacts, button condition and the spare key.
Range has become very short
A remote that used to work from several metres away but now only works beside the car commonly has a weak fob battery. Gradually decreasing range is an important clue.
Works intermittently
Intermittent operation can be caused by a weak battery, poor battery contacts, worn buttons, internal circuit damage or signal interference. Note whether moving or squeezing the key changes the symptom.
Only one button works
If lock works but unlock does not, or one individual button has stopped responding, suspect the button, switch contact or circuit board before blaming the vehicle receiver.
Manual key still works
If the mechanical blade opens the door normally but the remote does not operate the locks, the mechanical lock itself may be fine. Continue diagnosing the remote and central-locking functions separately.
One door does not respond
If the other doors lock normally but one door remains unlocked, the key fob may be doing its job. The fault may instead involve that door's actuator, latch, wiring or locking mechanism.
Central locking problems →Unlocks but will not start
The remote-locking command may be working while the vehicle is failing to recognise the key for starting. Look for a key-not-detected, immobiliser or security warning before assuming the remote itself is faulty.
Works in some places only
If the remote repeatedly fails in one location but works normally elsewhere, local radio-frequency interference becomes more plausible than a completely failed key.
Both keys stopped working
Two keys failing together changes the diagnosis. Check the vehicle battery, relevant fuses, locking system, receiver and body electronics before assuming both keys independently developed the same fault.
Describe the failure precisely. “My key does not work” is not enough information. Establish whether the fault affects lock, unlock, boot release, passive entry, manual entry, key detection or engine starting. The pattern usually narrows the fault faster than replacing parts.
One key fob not working vs both key fobs not working
Comparing the main key with the spare is one of the most valuable checks you can make. Two keys are independent pieces of hardware, but they communicate with the same vehicle. That makes the comparison extremely useful when deciding which side of the system deserves attention first.
Spare key works normally
If the spare key locks, unlocks and starts the car normally while the main key does not, concentrate first on the failed key.
- ✓ Check the fob battery.
- ✓ Confirm correct battery polarity.
- ✓ Inspect battery contacts.
- ✓ Compare button feel with the spare.
- ✓ Look for cracked or damaged casing.
- ✓ Consider previous water or impact damage.
- ✓ Investigate synchronisation or programming only if necessary.
Neither key operates normally
If two previously working keys develop the same problem, especially at roughly the same time, investigate the shared vehicle systems before buying replacement keys.
- ✓ Check vehicle battery condition and voltage.
- ✓ Check whether the central locking works from inside the car.
- ✓ Check relevant fuses where accessible.
- ✓ Look for other electrical symptoms.
- ✓ Check for key or immobiliser warnings.
- ✓ Consider receiver or body-control faults.
- ✓ Use diagnostic testing before condemning the keys.
A spare key that works correctly proves that the vehicle can still receive and process at least one authorised key. It does not prove every vehicle component is perfect, but it makes a fault inside the non-working key much more likely. If neither key works, the diagnosis should broaden towards systems shared by both keys.
It is possible for two fob batteries to be weak, especially if both keys are the same age, but identical simultaneous failure should not automatically be blamed on two batteries. If fresh, correctly fitted batteries do not restore either key, move the diagnosis towards the vehicle.
What causes a car key fob to stop working?
A non-working key fob does not have one universal cause. Problems can originate inside the key, between the key and the vehicle, or inside the vehicle itself. The most useful diagnosis combines the symptom pattern with the spare-key comparison rather than treating every fault as a flat battery.
Weak or flat fob battery
This is one of the most common and easiest faults to check. Early symptoms often include reduced range, intermittent operation or needing several button presses before the vehicle responds.
Wrong battery or polarity
A newly fitted battery does not guarantee the fob is powered correctly. A similar-looking but incorrect cell, reversed polarity or poor fit can leave the remote completely inoperative.
Poor battery contacts
Bent, contaminated, loose or damaged contacts can prevent a good electrical connection even when the battery itself is healthy. Intermittent operation after the key is moved or squeezed can be a useful clue.
Worn remote buttons
Frequently used buttons can wear internally. The rubber pad, switch or circuit-board contact may fail, causing one function to stop working while other buttons remain normal.
Damaged key casing
A cracked or loose case can allow movement, moisture and dirt into the fob. It can also prevent buttons from pressing the internal switches correctly.
Water or moisture damage
Washing a key, dropping it in water or allowing moisture to enter through a damaged case can corrode battery contacts and damage the internal circuit board.
Impact or circuit-board damage
Repeated drops can damage solder joints, switches, battery holders or circuit-board components even when the outside of the key still looks acceptable.
Lost remote synchronisation
Some vehicles use a remote-control synchronisation process that can be disrupted after certain battery or electrical events. The exact relearn procedure varies by manufacturer, so avoid applying an internet procedure from an unrelated vehicle.
Key programming issue
Remote synchronisation and immobiliser programming are not always the same thing. If the vehicle no longer recognises an authorised key for starting, specialist diagnostic or locksmith equipment may be required.
Radio signal interference
External radio-frequency interference can temporarily reduce remote range or prevent the vehicle receiving the signal. A key that fails in one location but works normally elsewhere is an important clue.
Weak vehicle battery
Low vehicle voltage can create unexpected behaviour in central locking, alarm, keyless-entry and immobiliser systems. If several electrical symptoms appeared together, do not diagnose the key in isolation.
Check car battery health →Central locking fault
The vehicle may receive the remote command correctly but fail to operate one or more locks because of an actuator, latch, wiring, fuse or control-system problem.
Central locking diagnosis →Remote receiver or aerial fault
The vehicle must receive the radio signal before it can act on the command. If neither remote works but the central locking can still be operated from inside the vehicle, the receiver side becomes more relevant.
Fuse or power-supply fault
Locking and body-control circuits need stable power and correct fuse protection. A fuse problem should be investigated alongside the reason it failed rather than simply replaced repeatedly.
Body control module fault
Modern vehicles often use a body control module or similar electronic controller to manage locking, alarm and convenience functions. Module faults are less common than batteries and damaged keys, so they should be diagnosed rather than guessed.
Immobiliser recognition fault
The doors may unlock normally while engine-start authorisation fails. A flashing security symbol, key-not-detected message or a spare key that starts the car normally can help separate an immobiliser problem from a remote-locking problem.
Car won't start but battery seems fine →Start with probability, but confirm with evidence. A weak fob battery is common, so it deserves an early check. A replacement battery that changes nothing is not a reason to keep changing batteries—it is a reason to move to the next branch of the diagnosis.
Key-side problems include the fob battery, contacts, buttons, casing, water damage and internal electronics. Vehicle-side problems include battery voltage, central locking, power supplies, remote reception, body-control electronics and immobiliser recognition. Separating those two groups early prevents unnecessary key replacement.
What should you check first when a key fob stops working?
Start with the simplest checks that give the most diagnostic information. The goal is not to replace parts immediately, but to separate an individual key fault from a vehicle-side electrical, locking or immobiliser problem.
Work through the checks in order where practical. If one step clearly identifies the fault, there is no benefit in jumping straight to programming, module replacement or a new key.
1. Try the spare key
If the spare key works normally, the fault is much more likely to be inside the non-working key. If both keys have the same symptom, investigate the vehicle side as well.
2. Check exactly what has failed
Test lock, unlock, boot release if fitted, passive entry, manual entry and starting. A partial failure often gives more useful information than a completely dead remote.
3. Check the fob battery
If one remote has poor range, intermittent operation or has stopped responding, check the battery type, condition, polarity and how firmly it sits against the contacts.
4. Inspect the key physically
Look for cracked casing, missing buttons, loose sections, signs of water entry or evidence that the key has been dropped or forced open.
5. Compare button feel
A button that feels unusually soft, hard, sticky or has no clear click may indicate damage to the internal switch, rubber pad or circuit-board contact.
6. Try the key in another location
If the remote fails in one particular place, move the car or try the key elsewhere before condemning the fob. Local signal interference can affect remote operation.
7. Check the manual key blade
If the vehicle has a physical emergency key, confirm that it can unlock the driver's door. This separates the mechanical lock from the remote-control function.
8. Check dashboard messages
Look for messages such as key not detected, key battery low, immobiliser active or steering-lock faults. These warnings can redirect the diagnosis away from simple remote locking.
9. Check the vehicle battery
If both keys fail or several electrical systems behave strangely, check the vehicle battery condition before blaming the keys.
How to check car battery health →10. Check internal central locking operation
If the locks operate normally from an interior switch but neither remote works, the problem may be closer to remote reception or key communication than the door actuators themselves.
The spare-key comparison should happen early, not at the end. It can save a great deal of unnecessary testing because it immediately tells you whether the problem follows one key or affects every authorised key.
How to diagnose a weak or flat key fob battery
A weak fob battery is common, but it should still be diagnosed properly. The most useful clue is usually a change in behaviour over time rather than an instant total failure.
Poorer range than before
If you previously unlocked the car from several metres away but now have to stand beside it, a weak battery is a strong first suspect.
Intermittent operation
A battery nearing the end of its useful life may operate the remote sometimes but not consistently.
Several presses needed
Repeatedly pressing the button before the car responds can indicate low battery output, although worn switches or contacts can produce a similar symptom.
Battery warning message
Some vehicles display a low-key-battery message before the remote fails. Treat the vehicle handbook as the correct reference for the battery type and replacement procedure.
New battery but still dead
Recheck the battery code, polarity and contacts. A newly purchased battery does not prove the fob is receiving power correctly.
Only one key affected
If the spare remote works perfectly, a battery or hardware fault in the affected key becomes much more likely.
Key fob battery replacement checks
- 1 Check the battery code on the original cell before buying a replacement.
- 2 Confirm the positive and negative sides are fitted in the correct orientation.
- 3 Make sure the battery sits securely against the internal contacts.
- 4 Inspect contacts for visible contamination, corrosion, distortion or damage.
- 5 Reassemble the case properly so the battery cannot move when buttons are pressed.
- 6 Test remote range, locking and engine-start recognition after replacement.
Key-fob cases often contain small clips, seals, circuit-board components and sometimes separate transponder pieces. If the correct opening method is unclear, use the vehicle handbook or manufacturer guidance rather than levering aggressively against the case.
If the correct battery is fitted properly and the symptom is unchanged, move on. Repeatedly changing coin cells will not repair a damaged button, broken circuit board, lost communication or vehicle-side fault.
Does a key fob need re-syncing or reprogramming?
Sometimes, but not every key-fob problem is a programming problem. The terms re-syncing, remote pairing and immobiliser programming are often used as though they mean the same thing, but they can refer to different functions.
Remote synchronisation
On some vehicles, the remote-control portion of the key may need to be re-synchronised with the vehicle after certain battery or electrical events. This mainly concerns remote locking communication.
Immobiliser programming
Engine-start authorisation may use a separate transponder or coded key-recognition process. Programming this system may require diagnostic, locksmith or manufacturer-specific equipment.
Battery replaced, remote dead
First confirm battery type, polarity and contacts. Only then consider whether the vehicle uses a re-sync procedure.
Remote works, car won't start
Do not focus only on remote pairing. The issue may involve immobiliser recognition, the transponder or another starting-system fault.
Replacement key added
A new or replacement key may need vehicle-specific programming before all functions operate correctly.
Procedures vary by manufacturer, model, year and key system. A method that works on one vehicle may be irrelevant to another. Use the handbook, manufacturer information or a competent automotive locksmith or diagnostic technician when programming is genuinely required.
Do not use “needs programming” as a catch-all explanation for every dead fob. First prove that the key has power, compare the spare, establish whether remote locking or engine recognition has failed, and only then decide whether synchronisation or programming belongs in the diagnosis.
Can signal interference or the vehicle receiver stop a key fob working?
Yes. For remote locking to work, the key must transmit a valid command and the vehicle must receive and process it. A problem anywhere in that path can produce the same basic complaint: pressing the button does nothing.
Local interference
Radio-frequency interference can reduce range or temporarily prevent remote operation. A fault that only occurs in one location is an important clue.
Vehicle receiver problem
If neither remote works but other central-locking functions remain available, the remote receiver or its supporting circuits may need investigation.
Power or module problem
Receivers and body-control systems depend on correct supply voltage, wiring and communication. Low voltage or electrical faults can therefore look like a failed key.
Signs that interference may be involved
- ✓ The remote works normally at home but repeatedly fails in one specific car park or location.
- ✓ Both keys are affected in the same location.
- ✓ Normal operation returns after the vehicle is moved.
- ✓ The manual key or emergency-entry method still works.
A receiver fault should be supported by the overall symptom pattern. If one key fails but the spare works normally, the receiver is capable of receiving at least one valid remote signal, which moves suspicion back towards the individual key.
Mechanic-style diagnostic process for a key fob fault
A good diagnosis moves from the driver's complaint to evidence. It does not begin with an expensive replacement key. The following workflow is designed to narrow the fault efficiently while avoiding unnecessary parts and programming.
1. Confirm the exact complaint
Establish whether the problem affects lock, unlock, boot release, passive entry, key detection, manual entry or engine starting.
2. Compare both keys
Determine whether the fault follows one key or affects all available keys. This is one of the strongest early diagnostic clues.
3. Inspect the affected key
Check the case, buttons, battery holder, visible contacts and any signs of moisture, impact or previous repair.
4. Verify the fob battery
Confirm the correct battery specification, orientation and connection rather than assuming a recently fitted battery must be good.
5. Check vehicle battery condition
If both keys fail or other electrical symptoms exist, confirm the vehicle has adequate voltage before diagnosing control modules or key-recognition systems.
6. Test central locking operation
Check whether the locks work from inside the vehicle and whether every door responds. This helps separate actuator faults from remote communication problems.
7. Check dashboard and security messages
A key-not-detected or immobiliser warning changes the diagnostic direction and may indicate a recognition fault rather than a basic locking problem.
8. Check relevant power supplies and fuses
Where appropriate and accessible, verify the circuits that support locking, receiver and body-control functions. Investigate repeated fuse failure rather than simply replacing the fuse again.
9. Scan the relevant vehicle systems
If basic checks do not locate the fault, diagnostic equipment may reveal stored faults in body-control, immobiliser, keyless-entry or related systems.
10. Test before replacing
Use the symptom pattern, measurements and scan information to prove whether the fault lies in the key, receiver, locking system, power supply or immobiliser before authorising expensive parts.
A fault code, warning message or non-working remote is evidence—not automatic proof that the most expensive named component has failed. Confirm the inputs, power supplies, communication and behaviour of the system before replacing the key or a control module.
If both authorised keys are not recognised, the vehicle will not start, security warnings remain active, a replacement key needs coding or body-control faults are suspected, an automotive locksmith, competent diagnostic technician or manufacturer-equipped specialist may be needed. Security systems are vehicle-specific and should not be bypassed or guessed at.
Key fob unlocks the car but the car will not start
A key that unlocks the doors is not automatically being recognised correctly for engine starting. Remote central locking and start authorisation can use different parts of the key and vehicle security system.
If the doors unlock normally but the dashboard reports key not detected, displays an immobiliser symbol or refuses to authorise starting, treat this as a key-recognition or starting-system problem rather than simply a remote-locking fault.
Key not detected
The vehicle is not recognising the key in the way required for starting. Follow the manufacturer's emergency-start procedure before assuming the key needs replacing.
Spare key starts normally
If the spare starts the vehicle immediately but the main key does not, the fault follows the individual key and deserves key-specific diagnosis.
Neither key starts the car
Broaden the diagnosis. Vehicle battery voltage, starting system, immobiliser, steering-lock system and body electronics may all need consideration.
What should you check?
- 1 Try the spare key if one is available.
- 2 Read any dashboard message exactly rather than assuming every no-start condition is caused by the key.
- 3 Check the vehicle handbook for the emergency key-detection or emergency-start position.
- 4 Check vehicle battery condition if electrical systems are slow, dim or behaving unusually.
- 5 If both keys are rejected, investigate the vehicle side before purchasing replacement keys.
A flat remote-control battery does not automatically mean the key's immobiliser function has failed. Vehicle designs differ, and many cars provide an emergency method for recognising the key when the fob battery is weak. Follow the procedure specified for the vehicle rather than using a generic online method.
Can a weak car battery stop the key fob working?
Yes. The key fob has its own small battery, but the vehicle's receiver, central locking, alarm, immobiliser and body-control systems depend on the car's 12-volt electrical supply.
If the vehicle battery voltage is too low, several electronic systems can behave abnormally at the same time. This is especially relevant when both keys appear to have stopped working, the car has recently needed a jump start or other electrical symptoms appeared at the same time.
Both keys affected
One weak key battery cannot explain why two independent keys suddenly produce the same vehicle-side symptom.
Electrical symptoms
Slow cranking, dim lighting, warning messages, resetting electronics or central-locking problems make vehicle battery condition more relevant.
Recent flat battery
If the vehicle battery has recently discharged, confirm its condition and charging system rather than assuming the key has independently failed.
Stable system voltage should be established before expensive electronic diagnosis. Low voltage can produce misleading symptoms and communication faults that disappear once the underlying battery problem is corrected.
What if both key fobs have stopped working?
When two previously working keys develop the same fault, investigate what they share: the vehicle. It is still sensible to confirm the fob batteries, particularly if both keys are the same age, but do not buy two replacement keys without checking the systems common to both.
Vehicle battery
Confirm adequate vehicle voltage, particularly if the car has recently been flat, jump-started or left standing.
Relevant fuses
Check the correct fuse information for the vehicle before testing. A repeatedly blown fuse requires diagnosis of the underlying circuit fault.
Central locking
Check whether the locks operate from the interior controls. This helps determine whether the problem affects the whole locking system or primarily remote operation.
Remote receiver
A receiver, aerial or supporting circuit problem can stop the vehicle responding to otherwise functional remote keys.
Body electronics
A body control module or related controller may manage locking, alarm and keyless-entry functions. Test inputs, power and communication before condemning the module.
Immobiliser recognition
If neither authorised key is recognised for starting, immobiliser or key-recognition diagnosis may require suitable diagnostic equipment.
If both existing keys stopped working together, replacing both without proving they are faulty risks spending several hundred pounds while leaving the actual vehicle-side fault untouched.
Should you repair, re-sync, reprogramme or replace the key?
Replacement is only one possible outcome. The correct repair depends on which part of the key or vehicle has actually failed.
Replace the fob battery
Appropriate when the battery is weak and the key itself is otherwise undamaged.
Re-case the key
A cracked or badly worn outer shell may sometimes be replaced while retaining serviceable internal components, depending on key design.
Repair buttons or electronics
Damaged switches, battery contacts or circuit-board faults may be repairable by a specialist rather than requiring complete replacement.
Re-synchronise remote
Suitable only when the remote has genuinely lost the synchronisation required by that particular vehicle.
Reprogramme the key
Programming may be necessary for replacement keys or certain recognition faults. Modern security systems often require specialist equipment and vehicle-specific access.
Replace the complete key
Appropriate when the key is lost, badly damaged, irreparable or when replacement is more economical than rebuilding the existing unit.
Repair the vehicle instead
If the keys test correctly, the actual repair may involve the vehicle battery, locking system, receiver, wiring, power supply or control system.
Do not throw an old key away simply because its buttons or case are damaged. Depending on the design, serviceable internal components, transponder parts or the mechanical blade may still be useful to the specialist carrying out the repair.
Can a broken key fob cause an MOT failure?
A non-working remote key fob is not, by itself, a general automatic MOT failure. What matters is whether the fault affects an item that is actually inspected during the test.
Door operation can matter
Driver and passenger doors must operate as required by the MOT inspection rules. Where the vehicle's relevant control for opening a door is a button or fob, failure of that system can become relevant if the door cannot be opened as required.
Anti-theft equipment can matter
The MOT manual includes specific inspection requirements for certain anti-theft devices, particularly steering-lock arrangements on applicable vehicles. This is different from simply testing whether the remote lock button works.
Do not treat “key fob not working” as an MOT result on its own. Ask what the fault prevents the vehicle from doing. A dead remote that does not affect required door operation or another tested item is different from a fault that leaves a required door unable to open correctly.
MOT standards can change. Check the current DVSA inspection manual if you need a definitive decision for a particular vehicle or testable defect.
How much does it cost to fix a car key fob in the UK?
Costs vary widely because a coin-cell battery, remote programming and a complete smart-key replacement are very different jobs. Vehicle make, key technology, security access, provider, location and whether all existing keys have been lost can all affect the final price.
The figures below are sensible UK planning figures rather than guaranteed quotes. Always confirm what the price includes, especially key cutting, programming, call-out charges and VAT.
Key fob battery
About £15
Current UK consumer cost data puts a straightforward key-fob battery replacement at around this level.
Buying Risk: LowKey programming
About £50–£250
Programming cost varies substantially with vehicle, security system and the equipment required.
Buying Risk: MediumManual replacement key
About £160 + VAT
A useful current UK planning figure for a basic manual replacement key.
Buying Risk: MediumTransponder replacement key
About £240 + VAT
Current UK averages are substantially higher than simple mechanical keys because electronic security components are involved.
Buying Risk: MediumRemote replacement key
About £320 + VAT
Current UK cost data places a typical remote replacement above a basic transponder or manual key.
Buying Risk: HighSmart or high-end key
£595+ possible
Premium smart keys can cost significantly more depending on manufacturer, security technology and availability.
Buying Risk: High| Problem | Likely first action | Cost exposure | Specialist needed? | Best next step |
|---|---|---|---|---|
| Poor remote range | Check correct fob battery | Low | Usually no | Replace battery correctly and retest range |
| One key dead, spare works | Diagnose individual key | Low to high | Sometimes | Check battery, contacts, buttons and key electronics |
| Damaged outer case | Assess whether re-casing is suitable | Usually lower than full replacement | Sometimes | Preserve original electronics where appropriate |
| Remote needs genuine programming | Vehicle-specific programming | Around £50–£250 | Usually yes | Use competent locksmith, diagnostic specialist or dealer |
| Complete remote key required | Cut, program and test replacement | Around £320 + VAT average | Yes | Compare complete quotes, not just blank-key price |
| Smart or premium key required | Vehicle-specific replacement | £595+ possible | Yes | Confirm exact VIN/key specification before ordering |
| Both existing keys fail | Diagnose vehicle first | Variable | Often | Do not buy two keys until the shared fault is proven |
| Unlocks but vehicle will not start | Key-recognition / starting diagnosis | Variable | Often | Check spare key, warnings and emergency-start procedure |
Ask whether the quote includes the physical key or fob, blade cutting where required, programming, testing, call-out charges and VAT. A low advertised key price may not represent the final fitted and programmed cost.
When one otherwise healthy key gradually loses remote range, checking the correct fob battery first is sensible. When both keys fail together, the same principle works in reverse: diagnose the shared vehicle systems before purchasing expensive replacement keys.
Common key fob diagnostic mistakes
Most unnecessary expense comes from replacing something before establishing which side of the system has failed.
- ! Buying a complete replacement key before checking the correct fob battery.
- ! Fitting the correct battery type with the polarity reversed.
- ! Assuming a brand-new battery proves the fob's internal power connection is good.
- ! Ignoring the spare key instead of using it as an immediate diagnostic comparison.
- ! Buying two replacement keys when both original keys fail at the same time.
- ! Treating remote locking and immobiliser recognition as exactly the same function.
- ! Assuming every key needs programming after a battery change.
- ! Following a programming procedure written for a different make, model or key system.
- ! Ignoring a weak vehicle battery when several electronic systems are malfunctioning.
- ! Condemning the body control module or remote receiver without testing power, communication and simpler causes.
- ! Assuming a non-working remote automatically means MOT failure.
- ! Comparing replacement-key quotes without checking whether cutting, programming, testing, call-out fees and VAT are included.
Start with the evidence that costs nothing: identify exactly what has failed, compare the spare key and check whether the fault follows one key or affects the vehicle. Then verify the simple electrical checks before escalating to programming, replacement keys or control-module diagnosis.
Should you buy a used car with a key fob problem?
A damaged key case or weak fob battery is very different from a vehicle that does not recognise either key. When inspecting a used car, test every supplied key before agreeing to buy and confirm that each key performs the functions it is supposed to perform.
Replacement remote and smart keys can be expensive, so a missing spare key or unresolved immobiliser problem should be treated as part of the vehicle's purchase cost rather than dismissed as a minor inconvenience.
Walk away or diagnose first if...
- ! Neither supplied key is consistently recognised.
- ! The vehicle displays persistent immobiliser or key-not-detected warnings.
- ! The vehicle will not start reliably with either key.
- ! Central locking and other body-electrical systems are malfunctioning at the same time.
- ! The seller says the fault “just needs programming” but cannot demonstrate the diagnosis.
Negotiate the price if...
- ✓ Only one working key is supplied.
- ✓ The remote range is poor but the key otherwise works.
- ✓ The outer case or buttons are visibly worn.
- ✓ A spare key is missing and replacement cost has not been allowed for.
- ✓ One remote function has failed but the vehicle starts normally and the second key works correctly.
Reassuring signs include...
- ✓ Two working authorised keys are supplied.
- ✓ Both keys lock and unlock the vehicle normally.
- ✓ Both keys start the vehicle without warning messages.
- ✓ All doors respond correctly to central locking.
- ✓ There are no unexplained immobiliser or security warnings.
Never test only the key the seller hands you first. Ask for every supplied key and test each one independently. A second key that has been sitting unused for years may have a weak battery, but a missing or unrecognised spare can also expose you to a meaningful replacement cost after purchase.
Confirm how many keys are included with the sale and whether every key starts the vehicle. If there is an unresolved immobiliser, central-locking or key-recognition problem, obtain a realistic repair or replacement estimate before deciding whether the vehicle still represents good value.
Key, Electrical & Starting Knowledge Centre
Key-fob faults often overlap with central locking, battery, immobiliser and no-start complaints. Use the guides below to follow the symptom that best matches what the vehicle is actually doing rather than replacing unrelated parts.
Key & Locking Problems
Battery & Charging
Starting Diagnosis
Diagnostic Tools
Used Car Buying
Motor Vehicle Expert Hubs
If the remote works but the engine does not start, move into the starting-diagnosis guides. If neither remote works but the car starts normally, stay with locking and key communication diagnosis. Choosing the correct symptom path prevents unrelated repairs.
Car Key Fob Not Working FAQs
These are some of the most common questions drivers ask when a remote key stops locking, unlocking or being recognised by the vehicle.
Why is my car key fob not working?
The most common cause is a flat fob battery, but lost sync, button damage, water damage, signal interference, vehicle receiver faults and immobiliser issues can also cause problems.
Why does my key fob only work close to the car?
Reduced range usually means the fob battery is weak. Replace it with the correct battery type and make sure it is fitted the right way round.
Why do both key fobs not work?
If both keys fail, suspect the vehicle battery, central locking system, receiver, fuse, body control module or immobiliser system.
Can I still unlock the car manually?
Usually yes, if the car has a hidden key blade or manual lock barrel. Some modern cars hide the key blade inside the fob.
Can a key fob stop the car starting?
Yes. The remote may unlock the car, but immobiliser recognition can still fail, especially if the key chip, battery, receiver or key recognition system has a problem.
Should I replace the whole key?
Not always. Try the correct battery first, then consider re-syncing, repair, re-casing, reprogramming or replacement if needed.
Can a weak car battery stop the key fob working?
Yes. Low vehicle voltage can affect body electronics, central locking, alarm and immobiliser systems.
Why does only one button work on my key fob?
A worn or damaged button, dirty contact or circuit board issue can cause one function to fail while others still work.
Can water damage ruin a key fob?
Yes. Moisture can corrode contacts or damage the circuit board. If the key got wet, it may need repair or replacement.
Can signal interference stop my key fob working?
Yes. If the key works in one location but not another, local interference may be affecting the remote signal.
What every driver should remember
Diagnose before spending
- ✓ Try the spare key early in the diagnosis.
- ✓ Check exactly which key functions have failed.
- ✓ Poor range often justifies checking the fob battery first.
- ✓ One failed key usually points more strongly towards that individual key.
- ✓ Both keys failing should move attention towards the vehicle as well.
Avoid these assumptions
- ! Do not assume every dead remote needs a complete new key.
- ! Do not confuse remote locking with immobiliser recognition.
- ! Do not assume every battery replacement requires programming.
- ! Do not replace control modules without proper diagnosis.
- ! Do not buy two replacement keys simply because both keys have stopped working.
If one key fails and the spare works normally, investigate the failed key first. If both keys fail, investigate the systems they share. That single comparison can prevent a large amount of unnecessary replacement and programming.
Fix the fault, not just the symptom
Most key-fob problems become much easier to diagnose once you stop treating the key as one single component. Remote locking, mechanical entry and engine-start authorisation can involve different parts of the key and different vehicle systems.
A weak remote battery may be a cheap and simple repair, while two keys failing together can point towards the vehicle battery, receiver, locking system or body electronics. A key that unlocks the doors but is not recognised for starting requires a different diagnostic path again.
Start with the symptom, compare the spare key, prove the simple causes first and only move towards programming, replacement keys or electronic modules when the evidence supports it. That approach gives you a better chance of repairing the actual fault without unnecessary expense.