P0304: Cylinder 4 Misfire Detected
Do not keep driving
Stop driving if the light is flashing. A steady light with rough running still warrants prompt diagnosis rather than waiting for the next service.
What this code means
Combustion is failing in cylinder 4. On a four-cylinder engine this is the cylinder at the gearbox end, often the one that runs hottest and collects the most carbon. Persistent misfires there that survive new plugs and coils are worth a compression and leak-down test before further parts are fitted.
Likely causes
Start here
- Faulty ignition coil · easy
- Worn or fouled spark plugs · easy
- Clogged fuel injectors · moderate
Less common
- Damaged spark plug wire or boot · easy
- Vacuum leak in intake manifold or hoses · moderate
Rare
- Low engine compression · specialist
- Burnt or leaking valve · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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