Engine Misfire: Causes, Likely Codes & What to Do
TL;DR
Engine misfire = a cylinder not firing correctly. Severity: high. STOP driving if the check engine light flashes. Most likely codes: P0300 (random misfire), P0301–P0308 (specific cylinder). Top causes: spark plugs, ignition coils, vacuum leaks. Scan to get the exact code.
What "engine misfire" means
Each cylinder fires in sequence to make power. A misfire is a failed or incomplete combustion event in one or more cylinders. You feel it as shaking, hesitation and lost power, and unburned fuel passes into the exhaust — which is why a severe misfire (flashing light) can quickly overheat and ruin the catalytic converter.
Quick diagnosis: IF → THEN
| If… | Then… |
|---|---|
| the check engine light is FLASHING | stop driving as soon as it is safe — an active misfire is damaging the catalytic converter |
| the shaking is worst at idle and smooths out when driving | suspect a single weak cylinder — spark plug or coil (scan for P030x) |
| misfire comes with a fuel/air lean code | suspect a vacuum leak or fuel-delivery issue rather than ignition |
| the misfire is random and moves between cylinders | suspect a shared cause: vacuum leak, fuel pressure, or timing (P0300) |
Most likely fault codes
| Code | Likelihood | Notes |
|---|---|---|
| P0300 — Random/multiple misfire | 50% | |
| P0301 — Cylinder 1 misfire | 30% | or P0302–P0308 for other cylinders |
| P0171 — System too lean (can cause misfire) | 20% |
Common causes
- Worn or fouled spark plugs
- Failing ignition coils or plug wires
- Vacuum / intake leak (lean misfire)
- Clogged or failing fuel injectors
- Low compression or mechanical fault
What to do
- Check whether the check engine light is flashing — if so, stop driving.
- Scan for trouble codes to identify the misfire type (P0300 vs P030x).
- Inspect spark plugs and swap-test coils on the affected cylinder.
- Check for vacuum leaks if a lean code is also present.
- Test fuel delivery and compression if ignition checks out.
When is it urgent?
A flashing check engine light means stop driving immediately — the converter is at risk. Steady shaking with lost power should be diagnosed the same day.
Frequently asked questions
Can I drive with an engine misfire?
Only briefly, and not at all if the check engine light is flashing. A sustained misfire dumps raw fuel into the exhaust and can destroy the catalytic converter.
What is the most common cause of a misfire?
Worn spark plugs, followed by failing ignition coils. Together they cause the majority of misfires.
How do I know which cylinder is misfiring?
Scan for codes: a cylinder-specific code like P0301 names the cylinder, while P0300 indicates a random or multiple-cylinder misfire.