P0303: Cylinder 3 Misfire Detected

What does P0303 mean?

Cylinder 3 is misfiring. On a four-cylinder engine it is one of the middle cylinders; on a V6 or V8 it may be on either bank depending on the maker's numbering. If P0303 shows up together with lean codes on the same bank, look for a shared cause like a vacuum leak rather than a single part.

Generic OBD-II definition: Cylinder 3 Misfire Detected. Confirm applicability in your vehicle service information.

Common causes

• Failing coil-on-plug unit for cylinder 3
• Carbon-tracked or gapped-out spark plug
• Restricted cylinder 3 injector
• Wiring chafed where the injector or coil harness runs over the engine
• Low compression in cylinder 3

Symptoms

• Vibration felt through the seat or steering wheel
• Misfire that gets worse in damp or rainy weather (common with cracked coil boots)
• Flashing check engine light under heavy throttle

Can I keep driving with P0303?

Severity: High Stop or minimize driving and get it looked at right away.

If the check engine light is flashing or the car runs badly, stop driving and have it checked.

Typical fixes

• Check the coil and injector connectors for corrosion or broken locking tabs
• Swap cylinder 3's ignition coil with a neighboring cylinder's coil, clear codes, and drive: if the code moves to that cylinder, the coil is bad
• If the misfire stays put, swap the cylinder 3 injector with a neighbor (on engines where that's practical) and see if it follows
• Inspect the coil boot for white tracking marks that show arcing