P0304 P0xxx · Powertrain (P) High severity
Cylinder 4 Misfire Detected
Cylinder 4 misfire. Swap-test ignition and fuel components first; compression last. This is the most reliable way to avoid replacing good parts.
Severity guidance: High — stop driving and diagnose now; risk of further damage
Symptoms you will notice
- Rough idle
- Shaking under acceleration
- Flashing MIL
- Poor fuel economy
Common causes
Ordered from most to least likely — start at the top.
- 1 Failed ignition coil
- 2 Fouled spark plug
- 3 Faulty injector
- 4 Low compression
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Confirm the cylinder with live misfire counters.
- ✓ Swap the coil and plug with a neighbouring cylinder.
- ✓ Swap the injector if ignition checks out.
- ✓ Compression test if the misfire does not move.
Repair options
- Replace ignition parts
- Service the injector
- Mechanical repair if compression is down
P0304 — frequently asked questions
Why does it only misfire when cold? +
A small head-gasket leak or a marginal valve seal often seals up as the engine warms and metal expands.
Should I replace all coils at once? +
If they share age and mileage, yes — the next one is usually not far behind.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.