P0304
Serious Severity (Level 4)Cylinder 4 Misfire Detected
OBD-II diagnostic trouble code P0304indicates a fault detected by the vehicle's onboard computer. This code applies to all OBD-II compliant vehicles and should be addressed promptly to avoid potential engine or emissions system damage.
Severity Level 4 — Serious
This code indicates a serious issue that can cause noticeable driveability problems and potential engine damage if not addressed. The vehicle should be taken to a mechanic as soon as possible. Extended driving is not recommended.
Common Symptoms
- Flashing or steady Check Engine Light
- Engine vibration, worse under load
- Rough idle and acceleration hesitation
- Loss of power
- Possible rotten egg smell as unburnt fuel reaches catalytic converter
Possible Causes
- Worn or fouled spark plug on cylinder 4
- Faulty ignition coil or bad spark plug wire for cylinder 4
- Clogged or leaking fuel injector on cylinder 4
- Low compression — burnt valve, worn rings, head gasket breach on cylinder 4
- Intake manifold gasket leak at cylinder 4 runner
- Carbon buildup on intake valves (direct injection engines)
Common Fixes
- Replace cylinder 4 spark plug ($10-30 DIY, $80-150 shop)
- Test and replace coil/wire for cylinder 4 ($50-200)
- Clean or replace cylinder 4 fuel injector ($80-400)
- Compression test — engine repair if low ($1500-4000)
- Walnut blast intake valves (DI engines, $400-800)
Estimated Repair Cost
$10 – $4000(typical range; varies by vehicle make and model)
These are estimated costs based on generic repair data. Actual costs may vary depending on your vehicle, location, and labor rates.
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