Cylinder 4 Misfire
P0304 means the powertrain computer detected a misfire on cylinder 4. It is a symptom code, not proof that one specific part failed.
A flashing check engine light with P0304 should be treated as urgent.
Common symptoms
- Check engine light
- Rough idle
- Shudder on acceleration
- Loss of power
- Fuel smell
Likely causes
- Ignition coil failure
- Worn spark plug
- Faulty injector
- Vacuum leak
- Low compression
What to check before replacing parts
Swap the coil and plug to another cylinder
Look for: A misfire that follows the parts to their new cylinder confirms one of them is bad. A misfire that stays on cylinder 4 points to the injector, wiring, or mechanical condition of that cylinder.
Inspect the wiring and connector at cylinder 4
Look for: Look for chafed insulation, corrosion, or a loose pin at the coil or injector connector — intermittent contact causes a misfire that can come and go with engine vibration or heat.
Check injector balance on cylinder 4
Look for: A cylinder that draws noticeably less (or more) fuel than the others on an injector balance/flow test points to a clogged or leaking injector rather than ignition.
Look for an intake leak near cylinder 4
Look for: A hiss at idle or a smoke-test leak at that runner or gasket lets in extra air, leaning out just that cylinder.
Run a compression or leak-down test
Look for: Compression well below the other cylinders, or leak-down air escaping the intake, exhaust, or oil filler, confirms a mechanical fault (valve, ring, or gasket) instead of an electrical or fuel one.
This page explains the generic P0304 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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