P0304: Cylinder 4 Misfire Detected
The engine computer has found that cylinder 4 is misfiring more than it should.
What P0304 means
With P0304, the misfire monitor has blamed cylinder 4. On an inline four, that is the cylinder at the far end from the timing cover, often near the transmission. The computer cannot tell why the cylinder missed, only that the crank did not speed up the way it should have on that cylinder power stroke. Fuel delivery is a bigger suspect than many people think: an injector at the end of the fuel rail or a cracked connector on the wiring harness can cut fuel to one cylinder. Shops check spark first with a swap test and a spark tester, then fuel with an injector balance or noid light test, then mechanics with a compression test. Fixing the true cause the first time matters, because replacing plugs on a car with a dead injector wastes money. Do not drive long with a misfire, since unburned fuel overheats the catalytic converter and can lead to a much bigger bill.
Common symptoms
- Check engine light on, or flashing when the misfire is bad
- Rough or shaky idle
- Hesitation or jerking when you accelerate
- Loss of power, especially going uphill or merging
- Worse gas mileage
Most common causes
- Bad spark plug or ignition coil on cylinder 4
- Fuel injector not opening or wiring fault to it
- Vacuum leak at the end of the intake manifold
- Damaged coil boot or plug wire
- Worn cam lobe or valve problem
- Low compression from worn piston rings
What to check yourself first
Swap the cylinder 4 coil with another cylinder and clear the code. With the engine running, carefully use a mechanic stethoscope or long screwdriver on the injector to listen for steady clicking. Check the injector and coil connectors for corrosion or broken locks.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace ignition coil | $238-$320 | The usual fix when the misfire follows the coil in a swap test. |
| Replace spark plugs | $288-$399 | Replace the full set if they are near their mileage limit. |
| Replace fuel injector | $768-$953 | If the injector does not click or fails a balance test. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For a broken injector or coil wire, common where harnesses bend near the back of the engine. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0300 Random/Multiple Cylinder Misfire Detected
- P0303 Cylinder 3 Misfire Detected
- P0301 Cylinder 1 Misfire Detected
- P0204 Injector Circuit/Open - Cylinder 4
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)