P0304 – Cylinder 4 Misfire Detected
What does P0304 mean?
The engine computer has detected a misfire in cylinder 4. On four-cylinder engines this is an end cylinder; on V6 and V8 engines its bank position varies by manufacturer. The fault is isolated to this cylinder's ignition, fuel delivery, or compression.
What causes P0304?
Listed from most to least common:
- 1Worn or fouled spark plug in cylinder 4
- 2Failing ignition coil on cylinder 4
- 3Clogged or failed injector on cylinder 4
- 4Vacuum leak at that intake runner
- 5Low compression from valve or ring wear
- 6Damaged spark plug wire
Code breakdown
Common symptoms
- Check engine light on or flashing
- Rough idle and noticeable shaking
- Loss of power and hesitation
- Poor fuel economy
- Possible fuel smell from unburned fuel
How to diagnose
- 1Locate cylinder 4 on your engine before starting work.
- 2Inspect the spark plug and compare it against the others.
- 3Swap the ignition coil to a neighboring cylinder to confirm or rule it out.
- 4Test the fuel injector by swapping or measuring resistance.
- 5Perform a compression test if ignition and fuel are good.
- 6Inspect intake gaskets near cylinder 4 for leaks.
Unburned fuel reaching the catalytic converter can destroy it. A flashing light indicates active severe misfire — stop driving.
Common fixes and estimated costs
| Repair | Estimated cost |
|---|---|
| Replace spark plugs | $100–$350 |
| Replace ignition coil | $100–$350 |
| Replace fuel injector | $150–$500 |
| Valve or ring repair | $1,200–$3,000 |
Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.
Good to know
Spark plugs and coils together account for the large majority of single-cylinder misfires. Start there before considering anything mechanical, and always replace plugs as a complete set.
Related codes
This information is provided for general reference only and may vary by manufacturer, model, and year. It is not a substitute for professional diagnosis or repair.