What Is P0306?
P0306 means the ECU has detected a misfire on cylinder 6. The ECU monitors each cylinder's contribution to engine rotation via the crankshaft position sensor — when cylinder 6 fails to fire or fires weakly, the resulting rotational hesitation is detected and P0306 is logged.
Cylinder 6 misfire codes appear on 6-cylinder (and larger) engines. The cause may be ignition-related, fuel-related, compression-related, or mechanical. Diagnosing which system is at fault is the key to an efficient repair — systematic diagnosis saves time and money compared to guessing at parts.
Common Symptoms
- Engine management light on — possibly flashing on severe misfires
- Rough or uneven idle
- Hesitation or stumbling under load
- Engine vibration consistent with cylinder 6 location
- Increased fuel consumption
- Possible catalytic converter damage (sulphur smell)
- Power loss, particularly under acceleration
Common Causes
How to Diagnose P0306
Swap Coil Pack First
On engines with individual coil packs, swap the cylinder 6 coil with one from a cylinder not showing a misfire. Clear the code and run the engine — if the misfire moves to the new cylinder, the coil is faulty. This single test is the quickest way to isolate an ignition fault.
Check Spark Plug Condition
Remove the cylinder 6 spark plug. A healthy plug has a light grey or tan electrode. Black (sooty) = rich mixture or misfire. White = lean mixture. Oil fouling = oil burning. Replace if worn or fouled.
Check Injector Operation
Swap the cylinder 6 injector with one from a non-misfiring cylinder (on fuel-injected engines). If the misfire follows the injector, it is faulty.
Compression Test
Fit a compression tester to cylinder 6. Normal compression is typically 150–200 psi (10–14 bar) depending on engine. Low compression compared to other cylinders indicates worn rings, valves, or head gasket failure.
Check for Vacuum Leaks
Spray carburettor cleaner carefully around the inlet manifold gasket near cylinder 6 with the engine running. A change in idle speed indicates a vacuum leak. Use smoke testing for a safer, more thorough check.
Check Fuel Pressure
Connect a fuel pressure gauge to the fuel rail. Incorrect fuel pressure affects all cylinders but can cause cylinder-specific misfires if the injector is borderline. Check pressure matches the manufacturer specification.
Verdict
Swap the coil pack first — it takes five minutes and diagnoses the most common cause instantly. If the misfire stays on cylinder 6 after the coil swap, move to spark plug, injector, and compression tests.
