P0304 Check Engine Code
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The problem
P0304 is misfire on cylinder 4 — on many four-cylinder engines, cylinder 4 is the rearmost plug and often the hardest to service; previous DIY tune-ups sometimes leave a loose boot or cracked plug there. Same ignition-first diagnosis as other cylinder-specific codes.
Symptoms
- P0304 on four-cylinder engines — very common isolated misfire code
- Rough idle, especially when cold
- May set with P0300 if counts spread
- After spark plug service where cylinder 4 is against the firewall
Can I keep driving with a P0304 code?
Gentle driving to the shop if the light is steady.
Flashing light — reduce throttle; misfire is active.
Common causes
- Spark plug or coil on cylinder 4
- Loose coil boot after recent plug change
- Injector on cylinder 4
- Compression loss
What it is often confused with
- P0300
- P0303
What happens if you ignore it
- Catalyst damage on short commutes
- Repeat misfire from loose boot
Diagnostic process
- 1 Cylinder 4 counts
- 2 Inspect plug and boot access
- 3 Coil test
What happens next at LugsNPlugs Automotive?
- 1 Mention recent plug work — cylinder 4 is often the one disturbed.
- 2 We verify counts on cylinder 4 before parts.
Common questions
- What does P0304 mean?
- P0304 is “Cylinder 4 Misfire Detected” — isolated misfire on the cylinder in the number 4 position for your engine’s firing order.
- Is P0304 common on 4-cylinder cars?
- Yes — on many inline fours, P0304 is a frequent isolated misfire code when one coil or plug fails. We still confirm on data, not assumptions.
Related problems
Colorado Springs independent repair — dealer-level diagnostics. Browse all common problems