P0303
P0303 Check Engine Code
Cylinder 3 Misfire Detected
Cylinder 3 is misfiring — combustion in that cylinder is incomplete enough for the computer to store P0303. On many V6 engines, cylinder 3 sits on bank 2, so lean codes like P0174 can ride along when that bank is short on fuel. Ignition on cylinder 3 (plug, coil, boot) is the first check; injectors and compression come after. Keep the load light if the light is flashing — unburned fuel still threatens the catalytic converter.
Symptoms
- Check engine light with P0303
- Rough running or shake at idle
- May appear with P0174 on bank 2 V6 layouts
- Flashing light when misfire is active under load
Can I keep driving with a P0303 code?
Steady light — schedule diagnosis; drive gently.
Flashing light — treat as urgent misfire; protect the catalytic converter.
Common causes
- Coil or plug on cylinder 3
- Bank 2 vacuum leak contributing to lean misfire
- Injector on cylinder 3
- Compression issue on cylinder 3
Often sounds like
- P0300
- P0174
- P0304
If you wait
- Converter damage from unburned fuel
- Bank 2 lean and misfire compounding
How we check it
- 1 Cylinder 3 misfire counters
- 2 Ignition inspection
- 3 Bank 2 fuel trim if lean codes are present
What happens next when you bring it in?
- 1 Call with engine type (I4, V6, V8) and any bank 2 lean codes.
- 2 We isolate cylinder 3 on live data before quoting parts.
Questions we get a lot
- What does P0303 mean?
- P0303 is Cylinder 3 Misfire Detected. The computer logged enough misfire events on cylinder 3 to store a code. On a V6, cylinder 3 is often on bank 2 — the side that does not include cylinder 1.
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