P0304: Cylinder 4 Misfire Detected
Misfires were detected in cylinder 4.
High risk
Drive gently and fix it soon. A flashing check engine light means stop driving under load.
Short answer · Can I keep driving with P0304?
Only with care. Drive gently and have it fixed soon. If the check engine light flashes, avoid heavy acceleration. Swapping the cylinder 4 coil with another shows whether the coil is at fault.
What does P0304 mean?
Code P0304 means cylinder 4 is misfiring. The cause is usually a part of that cylinder: spark plug, coil, injector, or a mechanical fault.
What the car does when the code is set
The check engine light comes on and may flash. Many engines switch off the injector of a cylinder that misfires heavily to protect the catalytic converter, so the engine can feel noticeably weaker and shake. Some cars also limit engine speed or power until the misfire stops.
How the system works
Every cylinder needs three things to burn its charge: the right mixture of air and fuel, enough compression and a strong spark at the right time. When one is missing, the cylinder misfires and the crankshaft slows down very slightly. The engine control unit measures the crankshaft speed many times per revolution and counts these small drops for each cylinder. If the misfire rate is high enough to raise emissions or to overheat the catalytic converter, it stores a code; at the higher level it also flashes the check engine light to warn that damage is possible.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Worn spark plugsPlugs past their service interval need more voltage to spark and start to misfire, often under load.
- Faulty ignition coil or plug leadA cracked coil or lead loses the spark, usually in one cylinder.
- Clogged or leaking injectorThe cylinder gets too little or too much fuel.
- Vacuum or intake leakUnmetered air makes the mixture too lean to burn reliably.
- Low compressionA burnt valve, worn rings or a head gasket fault stops the cylinder from producing power.
How it is diagnosed
- Note when it misfiresCold start, idle, under load or at high speed each point to different causes. The freeze-frame data shows speed, load and temperature when the code was set.
- Inspect plugs and coilsRemove the spark plugs and look for wear, oil or carbon. On engines with one coil per cylinder, swapping a coil with a neighbour shows whether the misfire follows the coil.
- Check fuel delivery and air leaksCheck injector operation and look for vacuum leaks with a smoke test.
- Run a compression testIf ignition and fuel are fine, a compression or leak-down test shows mechanical faults.If the check engine light is flashing, do not keep driving under load: the catalytic converter can overheat.
Questions about P0304
Can a misfire damage the engine?
Long-term misfiring can wash oil off the cylinder wall and overheat the catalytic converter. Fix it soon, and avoid heavy loads until then.
Can bad fuel cause misfires?
Yes, contaminated or wrong fuel can cause misfires in several cylinders. Usually, though, the cause is ignition, injection or a vacuum leak.
Why did the misfire start after a car wash or rain?
Water in the spark plug wells or on cracked coils and leads can short the spark. Drying the area may stop it, but cracked parts should be replaced.