What Is P0306?
P0306 is logged when the ECU identifies a misfire on cylinder 6. By monitoring crankshaft speed it can tell exactly which cylinder is stumbling, and a repeated failure on cylinder 6 sets this code. Cylinder 6 only exists on six-cylinder engines, so this is a V6 or straight-six fault.
Misfires are combustion faults rather than a single broken part, so the trail can lead to ignition, fuelling, or mechanical wear. On many V6 layouts cylinder 6 sits at the end of the rear bank, where heat and tight access mean its plug, coil, and wiring are easy to overlook until they cause trouble.
Common Symptoms
- Uneven, shaky idle
- Engine management light, often flashing
- Stumble or flat spot under acceleration
- Reduced power and response
- Worse fuel economy
- Raw fuel smell from the exhaust
Common Causes
How to Diagnose P0306
Find Cylinder 6
Identify cylinder 6 for your engine - typically the last cylinder on the rear bank of a V6 or the sixth along on a straight-six - so you test and swap the right parts.
Move the Ignition Parts
Swap cylinder 6's coil and plug with an adjacent cylinder, clear the code, and drive. A misfire that follows the parts confirms a faulty coil or plug.
Assess the Injector
Check that the cylinder 6 injector clicks and reads the correct resistance. A blocked or electrically faulty injector produces the same misfire as a weak spark.
Hunt for Air Leaks
Inspect the rear-bank intake gaskets and hoses for leaks. Extra air on cylinder 6 leans the mixture and is a common cause that the ignition checks will not reveal.
Compression and Leak-Down
If the spark and fuel sides are good, test cylinder 6's compression. A low result indicates a mechanical fault such as a burnt valve or worn rings.
Review Live Misfire Data
Watch the misfire counters and note the conditions - cold, hot, idle, or load. The pattern often distinguishes an ignition fault from a fuelling or mechanical one on cylinder 6.
Verdict
Work from cheap to dear: swap the cylinder 6 plug and coil first, then check the injector. If both are fine, look for a rear-bank air leak and finish with a compression test, since mechanical wear is the least likely but most serious cause.
Want the full picture? The OBD Fault Code Plain English Guide (PDF) covers the most common UK fault codes in one plain-English download.
