Quick answer
A misfire happens when one or more cylinders do not burn fuel properly. Common signs include rough idle, shaking, hesitation, jerking, loss of power, poor fuel economy, fuel smell, hard starting and an engine management light.
Common causes include spark plugs, ignition coils, fuel injectors, air leaks, compression problems, sensor faults, EGR faults, wiring problems, low voltage or overdue servicing.
A misfire with a flashing engine management light, strong shaking, fuel smell, smoke or severe power loss should be treated as urgent because it can damage the catalytic converter.
What does an engine misfire mean?
An engine needs the right mix of air, fuel, compression and spark at the right time. A misfire happens when that combustion does not happen properly in one or more cylinders.
From the driver’s seat, it can feel like shaking, stumbling, hesitation, jerking or a loss of power. From a mechanic’s side, the job is to find out whether the missing combustion is caused by ignition, fuel, air, compression or control-system faults.
Ignition problem
The spark is weak, missing or happening at the wrong time.
Fuel problem
The cylinder is not getting the right amount of fuel.
Air or compression problem
The air-fuel mixture is wrong, or the cylinder cannot seal properly.
Common engine misfire symptoms
Rough idle
The engine feels uneven, lumpy or shaky when stopped.
Vibration at idle →Hesitation
The car pauses, stumbles or feels delayed when accelerating.
Hesitation guide →Engine warning light
A misfire can trigger a steady or flashing engine management light.
Engine light guide →Loss of power
The car may feel weak, flat or reluctant to accelerate.
Power loss guide →Jerking under load
Misfires often show up when climbing hills, accelerating or carrying load.
Jerking guide →Poor cold running
Some faults are worst when the engine is cold, damp or first started.
Rough idle cold →Fuel smell
Unburnt fuel smell can appear if combustion is poor.
Petrol smell guide →Hard starting
Bad misfires can make the engine crank longer or struggle to start.
Cranks but won’t start →Bad fuel economy
The engine may use more fuel than normal because combustion is inefficient.
Find the closest symptom
Misfire at idle
Air leak, EGR fault, injector issue, ignition fault, compression issue or throttle body problem.
Idle vibration guide →Misfire under acceleration
Weak coil, worn plugs, fuel pressure, injector issue, boost leak or mixture problem.
Acceleration hesitation →Misfire when cold
Worn plugs, damp ignition parts, injector spray issue, coolant leak into cylinder or sensor reading fault.
Cold rough idle →Flashing engine light
Treat as urgent because an active misfire can damage the catalytic converter.
Flashing engine light →Misfire with smoke
Fuel, oil, coolant, turbo, injector or head gasket symptoms may be involved.
Smoke colour guide →Misfire with coolant loss
Possible head gasket, coolant entering a cylinder or internal engine issue.
White smoke coolant →Common causes of an engine misfire
Spark plugs
Worn, fouled or incorrect spark plugs can cause weak or inconsistent ignition.
Ignition coils
A failing coil can cause one cylinder to misfire, especially under load.
Fuel injector fault
A blocked, leaking or weak injector can make the engine run unevenly.
Air leak
Vacuum leaks or split intake hoses can upset the air-fuel mixture.
Sensor fault
Airflow, oxygen, crank, cam or temperature sensors can affect running.
Low compression
Internal engine wear, valve problems or head gasket issues can cause persistent misfires.
Head gasket symptoms →EGR fault
A sticking EGR valve can cause rough running, hesitation or poor idle.
Low voltage
Battery or charging problems can worsen ignition and sensor behaviour.
Battery warning guide →Neglected servicing
Old plugs, dirty filters, poor oil condition and poor maintenance can contribute.
Servicing guide →When the misfire happens gives useful clues
Misfire at idle
Often linked to air leaks, ignition faults, injector issues, EGR faults, compression problems or throttle body issues.
Misfire under acceleration
Commonly points towards ignition coils, spark plugs, fuel delivery, boost leaks or load-related faults.
Misfire when cold
Can be caused by worn plugs, weak coils, injector spray pattern issues, coolant entering a cylinder or sensor readings when cold.
Misfire when hot
Heat can expose failing coils, sensors, wiring, fuel pressure issues or internal mechanical problems.
Misfire after rain or washing
Moisture can affect ignition leads, coil packs, plugs or electrical connectors.
Misfire with white smoke
White smoke and coolant loss can suggest coolant entering a cylinder and needs urgent checking.
White smoke coolant guide →Petrol vs diesel misfire symptoms
Petrol engine misfire
Petrol misfires are often linked to spark plugs, ignition coils, injectors, air leaks, compression issues, oxygen sensors or fuel mixture faults.
Diesel engine misfire
Diesel misfire-like symptoms may involve injectors, glow plug systems, compression, EGR faults, DPF problems, turbo issues or fuel pressure problems.
DPF warning guide →A diesel may not feel like a petrol misfire, but rough running, smoke, knocking, poor starting or loss of power still needs proper diagnosis.
When an engine misfire is urgent
- !The engine management light is flashing.
- !The car shakes heavily or feels unsafe.
- !There is severe power loss or limp mode.
- !You smell fuel or see smoke.
- !The engine is overheating.
- !The misfire gets worse quickly.
- !The car struggles to pull away safely.
- !Other warning lights appear at the same time.
A flashing engine management light can mean the misfire may damage the catalytic converter. Avoid hard driving and arrange diagnosis quickly.
For safe driving advice, read can you drive with engine management light on?.
What to check next
1. Notice the pattern
Cold only, hot only, idle only or acceleration-only symptoms help narrow the cause.
2. Check warning lights
Fault codes can identify which cylinder or system is involved.
3. Avoid random parts
Misfires have several possible causes, so guessing can waste money.
4. Check service history
Old plugs, overdue servicing or neglected maintenance can contribute.
5. Confirm spark and fuel
A garage may check ignition, fuel delivery, compression and sensor data.
6. Do not ignore flashing lights
A flashing warning light should be treated as urgent.
How garages diagnose a misfire
A fault code can point to a cylinder or system, but it does not always prove the failed part. A good diagnosis checks the evidence before replacing parts.
Fault code scan
Looks for cylinder misfire codes, fuel trim codes, sensor codes or emissions faults.
Live data checks
Fuel trim, oxygen sensor, airflow, temperature and misfire counters can guide diagnosis.
Ignition checks
Spark plugs, coils, leads and wiring may be checked or swapped between cylinders.
Fuel checks
Injector operation, fuel pressure and delivery may be checked if spark is not the issue.
Compression test
Used when a misfire stays on one cylinder or does not respond to ignition/fuel checks.
Smoke or leak test
Can find air leaks that make the engine run lean and misfire.
Possible UK repair costs
Misfire repair cost depends on the cause. A plug or coil fault is very different from injector, compression or internal engine problems.
Spark plugs
Usually one of the more straightforward misfire repairs if access is simple.
Ignition coil
Common on petrol cars and often diagnosed by cylinder-specific misfire codes.
Injector repair
Can cost more, especially on diesel engines or hard-to-access injectors.
Air leak repair
Cost depends on whether it is a simple hose, intake gasket or hidden leak.
Sensor fault
Varies depending on sensor type, location and diagnostic time.
Compression problem
Can become expensive if valves, piston rings or head gasket faults are involved.
For wider budgeting, see car repair costs guide UK.
Common mistakes drivers make
- !Driving hard when the engine management light is flashing.
- !Replacing spark plugs without checking coils, injectors or compression.
- !Clearing fault codes before recording them.
- !Ignoring fuel smell, smoke or strong shaking.
- !Assuming every misfire is a cheap ignition fault.
- !Leaving a misfire until MOT or service day.
Best mechanic-style advice
Do not keep driving a car that is badly misfiring just because it still moves. A misfire can send unburnt fuel into the exhaust, damage the catalytic converter and make a simple ignition repair much more expensive.
Also do not throw parts at it blindly. Find out whether the fault follows the spark plug, coil, injector, wiring or cylinder itself. That is how you avoid replacing good parts and missing the real fault.
Related engine and diagnostics guides
Frequently asked questions
Can I drive with an engine misfire?
Not recommended if the car shakes badly, loses power or the engine light flashes. A serious misfire can damage the catalytic converter.
Can a misfire damage the engine?
Yes, if ignored. It can also damage the catalytic converter, especially if unburnt fuel enters the exhaust.
Can bad spark plugs cause a misfire?
Yes. Worn, fouled or incorrect spark plugs are a common cause of misfires on petrol engines.
Can a misfire come and go?
Yes. Heat, moisture, engine load and failing parts can make misfire symptoms intermittent.
Will a misfire show a fault code?
Often yes, but not always. Live data and proper testing may still be needed.
Is a flashing engine light serious?
Yes. It can indicate an active misfire and possible catalytic converter damage risk.
Can a diesel engine misfire?
Diesel engines can run rough, knock, smoke or lose power because of injector, fuel pressure, compression, EGR, DPF or glow plug issues.
Why does my car misfire only when accelerating?
Load-related misfires often point towards ignition coils, spark plugs, fuel delivery, boost leaks or air-fuel mixture problems.
Can a bad battery cause misfire symptoms?
Low voltage or charging issues can affect ignition, sensors and engine control, so battery and alternator condition should be checked if electrical symptoms are present.
Should I replace all spark plugs or only one?
If spark plugs are worn or overdue, they are usually replaced as a set. A garage should still confirm the fault before replacing parts.