Engine Misfire Symptoms
Check soon — immediately if the check engine light flashes
An occasional subtle misfire is a get-it-diagnosed-this-week problem. A constant misfire, or one that makes the check engine light flash, risks the catalytic converter and should stop you from driving.
What it means
A misfire is one or more cylinders failing to produce power on their combustion stroke — no spark, no fuel, or no compression at the moment it mattered. You feel it as a stumble, shudder, or rhythmic shake, especially at idle or under acceleration; the exhaust may pop or smell of fuel; fuel economy drops.
The engine computer counts misfires per cylinder and logs codes (P0301 = cylinder 1, P0302 = cylinder 2, and so on; P0300 = random/multiple). That cylinder number is the single most useful piece of diagnostic information — it turns 'the engine misfires' into 'test coil and plug on cylinder 3'.
Likely causes
| Cause | Likelihood | Severity |
|---|---|---|
| Worn or fouled spark plugs The default suspect, especially past the replacement interval. | Most common | Moderate |
| Failing ignition coil Classic single-cylinder misfire on coil-on-plug engines. Swapping the coil to another cylinder to see if the misfire follows is a standard test. | Common | Moderate |
| Fuel injector problem Clogged, sticking, or electrically dead. | Less common | Moderate |
| Vacuum leak affecting one cylinder An intake gasket leak near one runner leans that cylinder. | Less common | Moderate |
| Low compression (valve, rings, head gasket) The mechanical cause — usually persistent, unaffected by new plugs/coils, confirmed by a compression test. | Rare | Serious |
DIY checks before the shop
- Read the codes: the cylinder number in P030X codes directs the whole diagnosis.
- Check the plugs' service history — if they're overdue, start there.
- Note the conditions: misfire under load only (ignition breaking down under pressure), cold only, or constant.
- If comfortable, swap the suspect cylinder's coil with a neighbour and see if the misfire code follows the coil.
- Watch for the flashing check engine light — that's the signal to stop driving.
What repairs typically cost
| Repair | Typical range |
|---|---|
| Spark plugs (set, shop) Cheapest common fix. | Roughly $100–$400 |
| Ignition coil (each, shop) Rear-bank coils on transverse V6s cost more in labor. | Roughly $100–$300 |
| Fuel injector (each, shop) Varies by engine. | Roughly $150–$500 |
| Compression-related repair Head gasket, valve, or ring work is major surgery — get a confirmed diagnosis (compression/leak-down test) before going here. | Roughly $1,000 to several thousand |
Ranges are broad US-typical estimates for common vehicles at independent shops. Luxury brands, dealers, and regional labor rates can land well outside them — treat these as orientation, not quotes.
Rev knows your specific vehicle and service history — describe the symptom and get advice for your car, not a generic answer.
Frequently asked questions
What does a misfire feel like?
A stumble or shudder — like the engine briefly loses a beat. At idle it's a rhythmic shake; under acceleration it's hesitation or jerking, sometimes with popping from the exhaust.
Can I drive with a misfire?
Gently and briefly, if the check engine light is solid and the misfire is intermittent. If the light flashes, no — unburned fuel is cooking the catalytic converter as you drive.
Why does it misfire only in the rain or damp weather?
Moisture finds weaknesses in ignition components — cracked coil boots and worn plug insulators leak spark when damp. Damp-weather misfires point strongly at ignition.
