P0303 Check Engine Code
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.
Often confused with
- P0300
- P0174
- P0304
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
If you ignore it
- Converter damage from unburned fuel
- Bank 2 lean and misfire compounding
Repair overview
- Cylinder 3 misfire counters
- Ignition inspection
- Bank 2 fuel trim if lean codes are present
What happens next
- Call with engine type (I4, V6, V8) and any bank 2 lean codes.
- We isolate cylinder 3 on live data before quoting parts.
Questions
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.