P0303 Check Engine Code
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The problem
P0303 means repeated misfires on cylinder 3. On V6 engines, cylinder 3 is often on bank 2 — lean codes on that bank may accompany the misfire. Ignition testing on cylinder 3 comes first.
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
What it is often confused with
- P0300
- P0174
- P0304
What happens if you ignore it
- Converter damage
- Bank 2 lean and misfire compounding
Diagnostic process
- 1 Cylinder 3 misfire counters
- 2 Ignition inspection
- 3 Bank 2 fuel trim if lean codes present
What happens next at LugsNPlugs Automotive?
- 1 Note engine type (I4, V6, V8) and any bank 2 lean codes.
- 2 We isolate cylinder 3 before quoting parts.
Common questions
- What does P0303 mean?
- P0303 is “Cylinder 3 Misfire Detected” — the computer logged enough misfire events on cylinder 3 to store a code.
Related problems
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