Stop if MIL flashing. Steady MIL: repair within 1-2 days. P0306 means the ECM detected a misfire on cylinder 6 confirmed by crankshaft speed irregularity analysis. In V6 engines, cylinder 6 is the rearmost cylinder on bank 2 and the hardest to access for maintenance, making neglected spark plugs and corroded coil connectors common culprits.
What P0306 means
The ECM monitors crankshaft speed variations every 200 and 1000 revolutions. When cylinder-6 misfire events exceed the emission threshold (approximately 1.5 percent misfire rate over 1000 revolutions) or the catalyst-damage threshold (77 or more misfires per 200 revolutions), P0306 sets. Two-trip detection applies for the emission level; the catalyst-damage level triggers an immediate MIL flash. Cylinder 6 is the rearmost in a V6 firing order, making it prone to neglected spark plug service and coil connector corrosion caused by coolant proximity, oil intrusion at the valve cover, and limited access during routine maintenance.
Symptoms
- MIL illuminated steady or flashing (flashing means the catalyst-damage threshold is active)
- Rough idle or stumble, especially at low rpm in gear
- Power loss or hesitation at the rpm and load stored in freeze frame
- Raw fuel smell from exhaust
- Possible white exhaust smoke or unexplained coolant loss if a head gasket breach at cylinder 6 is the cause
Common causes
- Worn or fouled spark plug in cylinder 6 -- the rear position makes it prone to neglect; inspect electrode gap, oil fouling, and carbon deposits
- Failed coil-on-plug ignition coil on cylinder 6 -- connector corrosion from coolant or oil at the rear of the cylinder head
- Clogged or faulty fuel injector on cylinder 6 -- a restricted injector leans out the last cylinder in the bank
- Low compression from a burned exhaust valve or worn piston rings
- PCV system blockage or disconnected hose -- elevated crankcase pressure forces oil into the intake, fouling the cylinder-6 spark plug
- Engine coolant temperature sensor error -- inaccurate ECT causes incorrect cold-start fuel delivery that may manifest on the last cylinder first
- Cam phaser or timing chain fault on bank 2 -- retarded intake cam timing reduces effective compression at specific rpm
Severity & driving advice
Severity: high — Flashing MIL = catalyst-damage rate active; stop safely. Steady MIL: same-week repair required to prevent catalytic converter damage.
Can I drive? Stop if MIL flashing. Steady MIL: repair within 1-2 days.
Diagnostic approach
Make & model notes
:
:
FAQ
Why does P0306 appear only at idle and not under load?
An idle-specific misfire on cylinder 6 often points to a slightly clogged fuel injector that cannot deliver adequate fuel at low pulse-width (idle fuel demand), or a compression problem that is most apparent at low rpm when cylinder-filling efficiency is critical. Under load, the injector pulse-width widens and partially compensates for the restriction.
Can a head gasket cause P0306?
Yes. A head gasket breach between cylinder 6 and a coolant gallery allows coolant into the combustion chamber, causing rough combustion and typically white exhaust smoke. Confirm with a compression test and a combustion-gas leak test using block-test dye in the coolant reservoir.
Is cylinder 6 always on bank 2?
In most V6 engines yes -- cylinder 6 is on bank 2, opposite the bank containing cylinder 1. In inline-6 engines, cylinder 6 is at the rear of the block. Check your engine's cylinder-numbering diagram; assignments vary by manufacturer and engine layout.