Use the diagnostic app for key and electrical faults
You can use the free Motor Vehicle Expert diagnostic app to compare key fob problems, central locking faults, immobiliser warnings, weak battery symptoms and no-start issues.
Check what still works
Remote locking, manual key blade, spare key, starting and dashboard warning messages all give useful clues.
Separate key fault from car fault
One faulty key usually points towards the fob. Both keys failing often points towards the vehicle.
Understand immobiliser clues
A key may unlock the car but still fail to be recognised for starting.
Choose safer next steps
Check battery, sync, buttons, vehicle voltage and central locking before replacing expensive keys.
Quick answer
The most common reason a car key fob stops working is a flat or weak battery inside the fob. If the range has reduced gradually, the buttons only work close to the car, or the remote works sometimes and not others, replacing the key battery is usually the best first step.
If both keys fail, the issue is more likely to be with the vehicle battery, central locking system, receiver, fuse, body control module or immobiliser system rather than one individual key.
One key not working usually points towards the key. Both keys not working usually points towards the car, its battery, receiver, locking system or immobiliser.
Common reasons a car key fob stops working
Flat fob battery
Usually causes poor range first, then intermittent operation, then total remote failure.
Lost sync
Some keys need re-syncing after battery replacement, long battery disconnection or electrical faults.
Worn buttons
Buttons may stop clicking properly or only work when pressed hard.
Water damage
Moisture can damage the circuit board, battery contacts or internal components inside the key.
Signal interference
Remote locking may fail in certain locations due to local radio interference or blocked signal.
Central locking fault
If the key sends a signal but the locks do not move, the car may have a locking issue.
Central locking guide →Weak car battery
Low vehicle voltage can affect body electronics, locking systems and immobiliser behaviour.
Battery keeps going flat →Receiver fault
If no remote keys work, the car’s receiver, aerial or body control module may be involved.
Immobiliser issue
The car may unlock but fail to recognise the key for starting.
Car won’t start guide →What still works?
Before buying a new key, check the pattern. A key fob problem is not always the same as a central locking problem or immobiliser problem.
Remote does nothing
Start with the fob battery, spare key, button feel and possible water damage.
Only range is poor
A weak fob battery is very likely, especially if the range has reduced gradually.
Manual key works
The mechanical lock may be fine while the remote side of the key has failed.
Both keys fail
Suspect vehicle battery, receiver, fuse, control module, immobiliser or central locking fault.
Unlocks but will not start
The immobiliser or key recognition system may be the issue.
One door does not lock
That points more towards a door actuator, wiring or central locking fault.
Door lock guide →What to check first
- 1Try the spare key if you have one.
- 2Replace the key fob battery with the correct type and polarity.
- 3Check whether the buttons feel broken, sticky, loose or unusually soft.
- 4Try locking and unlocking in a different location in case of signal interference.
- 5Check whether the manual key blade unlocks the driver’s door.
- 6Check for dashboard key, immobiliser, steering lock or battery warning messages.
- 7If both keys fail, check the vehicle battery and central locking system.
- 8If the car unlocks but will not start, check the emergency start procedure in the handbook.
Key fob battery replacement tips
Use the correct battery
Check the old battery code before fitting a replacement. Similar-looking batteries are not always the same.
Fit it the right way round
Incorrect polarity can stop the fob working even with a new battery.
Avoid damaging the case
Use care when opening the fob so clips, seals and the circuit board are not damaged.
Clean visible contacts carefully
Dirty or bent battery contacts can stop the fob receiving power.
Re-sync if needed
Some cars need a simple key sync procedure after battery replacement.
Test both functions
Check locking range and whether the car still recognises the key for starting.
Key fob unlocks but car will not start
If the fob unlocks the car but the engine will not start, the remote locking part may be working while the immobiliser or key recognition side has a fault.
Some vehicles need the key placed in a specific emergency start position when the fob battery is weak. This may be near the steering column, start button, centre console or another marked area depending on the vehicle. Check the handbook, then investigate immobiliser warnings if the problem continues.
- ✓The doors unlock but the dashboard says key not detected.
- ✓The start button does nothing or asks you to hold the key in a certain place.
- ✓The immobiliser or key symbol flashes.
- ✓The spare key starts the car but the main key does not.
If both key fobs stop working
When both keys fail together, it is less likely that both key batteries failed at exactly the same time. The vehicle side should be checked, especially if the car battery has recently gone flat, electrical work has been done, or the central locking has other symptoms.
Vehicle battery voltage
Low voltage can upset locking, alarm, immobiliser and body control systems.
Battery health check →Central locking fault
Lock actuators, fuses, wiring or control modules may stop the locks responding.
Central locking guide →Receiver or module issue
The car may not be receiving or processing the key signal correctly.
Repair or replace the key fob?
If the fob works after a battery change, no further repair is usually needed. If buttons are damaged, the case is cracked, the circuit board is water damaged or the key has lost programming, a repair or replacement key may be needed.
Dealer replacement keys can be expensive, but automotive locksmiths may be able to repair, re-case, reprogram or replace keys depending on the vehicle. Avoid throwing away the old key until you know whether the blade, chip or circuit board can be reused.
Common mistakes drivers make
- !Buying a full replacement key before trying a correct battery.
- !Fitting the fob battery the wrong way round.
- !Assuming the key is faulty when both keys fail at the same time.
- !Ignoring a weak vehicle battery that is affecting electronic systems.
- !Forcing the key case open and damaging the circuit board.
- !Confusing a remote locking fault with an immobiliser no-start fault.
Best mechanic-style advice
Start simple. Try the spare key, replace the fob battery correctly, test the manual blade and check whether the car starts. Those checks quickly separate a weak key battery from a vehicle-side fault.
If both keys fail, do not rush to buy two new keys. Check the vehicle battery, central locking, fuses, receiver and immobiliser system first. If one key fails but the spare works normally, the fault is much more likely to be with that individual key.
Related ownership and electrical guides
Frequently asked questions
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.