P0304

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

These are estimated costs based on generic repair data. Actual costs may vary depending on your vehicle, location, and labor rates.

Explore Repair Costs

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