P0303 Cylinder 3 Misfire Detected

P0303 indicates a detected misfire on cylinder 3. The named cylinder narrows the search, but it does not identify the failed part.

Code details

Generic OBD-II · P0

System: Ignition / Misfire
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not clear and ignore repeated cylinder 3 misfires; recurring misfire can damage the catalyst and mask mechanical problems.

Symptoms of P0303

Rough idle or a distinct shake, often worse at low RPM.
A stumble or dead spot traced to cylinder 3.
A flashing check-engine light when the misfire is active.
Slight power loss and worse fuel economy.

Common causes of P0303

A worn spark plug or failed ignition coil on cylinder 3.
A clogged, leaking, or dead injector on cylinder 3.
A vacuum leak or low compression local to that cylinder.
Carbon-tracked plug boot or damaged coil wiring.

Useful checks before replacing parts

Compare cylinder 3 data with other cylinders using misfire counters, contribution data, or balance tests.
Inspect plug, coil or wire, injector, compression, valve sealing, and local intake sealing.
Use freeze-frame RPM and load to decide whether the fault is idle-only, load-related, or constant.

Related OBD-II codes

These nearby ignition / misfire entries can help compare wording and pin down the fault area.