P0306: Cylinder 6 Misfire Detected
The engine control unit has counted repeated misfires in cylinder 6.
High risk
Reduce speed and avoid heavy loads. If the check engine light flashes, the catalytic converter can overheat.
Short answer · Can I keep driving with P0306?
Only with care. If the light is steady, drive gently to a workshop soon. If it flashes, avoid acceleration and heavy loads.
What does P0306 mean?
Code P0306 means cylinder 6 is not burning its mixture properly, on engines with six or more cylinders. Check the coil, spark plug and injector of that cylinder first.
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 P0306
Can a leaking valve cover gasket cause P0306?
Yes. Oil in the spark plug well can cause arcing and misfire, especially on engines where the plugs sit under the valve cover.
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.