Symptoms
- Check engine light on or flashing with a specific cylinder code
- Rough idle or a distinct miss you can feel
- Possible popping sound from the exhaust
- Loss of power under load
Likely causes
- CommonBad spark plug on that cylinderA fouled, worn, or cracked plug in one spot causes a localized misfire.
- CommonFailing ignition coil on that cylinderCoil-on-plug systems have one coil per cylinder; one can fail while others are fine.
- PossibleClogged or failed fuel injector on that cylinderA stuck or clogged injector starves just that cylinder of fuel.
- PossibleLow compression in that cylinderA burnt valve or worn rings in one cylinder can't be fixed with parts swaps alone.
- Less commonDamaged spark plug wire (on wire-type ignition)Older systems with plug wires can have one cracked or arcing wire.
What to do
- A flashing light means stop driving soon; a steady light can usually wait for a scheduled repair.
- Note the exact cylinder number from the code, since it tells you exactly where to look.
- Swap the spark plug and ignition coil from the misfiring cylinder with a neighboring one; if the misfire follows the part, that's your culprit.
- Inspect the spark plug for fouling, damage, or incorrect gap.
- Replace the confirmed bad plug or coil and clear the code, then retest with a drive cycle.
- Have compression tested by a shop if swapping plug and coil doesn't move or fix the misfire.
Tools and parts
- OBD-II code reader
- Spark plug socket and ratchet
Spark plugSearch: “spark plug for [year make model] cylinder”
Ignition coilSearch: “ignition coil for [year make model]”
When to call a pro
See a shop for a compression test if swapping the plug and coil doesn't resolve it, since that points to a mechanical issue inside the engine.
Safety
Use caution
A flashing check engine light during a misfire means reduce speed and get it checked soon to avoid catalytic converter damage from unburned fuel.