P0306 means cylinder 6 specifically is misfiring, which narrows the search to that cylinder’s spark plug, ignition coil, injector or compression instead of the whole engine. If P0305 is stored at the same time, the cause is more likely something the two cylinders share rather than two separate failures. Cylinder 6 only exists on six- and eight-cylinder engines, and its location varies by manufacturer.
What Does P0306 Mean?
The engine computer detects a misfire by watching tiny changes in crankshaft speed, and P0306 is the code for cylinder 6 not firing properly. The background on misfires and their effect on the catalytic converter is in our P0300 guide. Its single-cylinder siblings include P0304 and P0305.
If P0305 and P0306 are both stored, look for a shared cause first: the two cylinders may sit on the same bank, share a coil wiring branch or connector, be fed by the same injector harness, or be affected by a head gasket leak between them. Two unrelated plug failures at the same time is less likely.
Common Symptoms
- Check engine light on, flashing when the misfire is severe — see our flashing check engine light guide.
- Rough idle and a shake under acceleration — see car shakes when accelerating.
- Hesitation and reduced power when you press the throttle.
- Worse fuel economy.
Common Causes, Most Likely First
- A worn or fouled spark plug in cylinder 6 — see our how often to replace spark plugs guide.
- A failed ignition coil or damaged connector on that cylinder.
- A clogged or failing injector for cylinder 6.
- A vacuum leak near that cylinder’s intake runner.
- Low compression in cylinder 6, or a head gasket leak affecting it — the most serious cause.
How to Diagnose It
- Confirm which physical cylinder is number 6 on your engine from its layout diagram.
- Swap the coil with a neighbouring cylinder’s, clear the code and drive. If the code moves, the coil is at fault.
- Inspect the plug and replace it if it is worn or fouled.
- Check the injector if the coil and plug are good.
- Run a compression test if electrical and fuel causes are ruled out.
Possible Fixes
| Cause | Fix | Typical cost |
|---|---|---|
| Worn spark plug | Replace plug(s) | $40 – $200 |
| Failed ignition coil | Replace the coil | $100 – $300 |
| Clogged or failing injector | Clean or replace the injector | $150 – $450 |
| Vacuum leak | Replace the hose or gasket | $50 – $350 |
| Low compression / head gasket | Engine repair | $500 – $3,500+ |
Can I Drive With P0306?
Short trips only, and gently. Continued misfiring can overheat the catalytic converter — see P0420 for how that damage shows up — and a flashing check engine light should be treated as urgent, per our flashing light guide.
When to See a Mechanic
As soon as possible if the light flashes, the engine runs rough or you also see P0305. Mention that the code is for a single cylinder. More in our engine section.
Frequently Asked Questions
What does P0306 mean?
Cylinder 6 is misfiring. It points to that cylinder’s plug, coil, injector or compression.
What if I have P0305 and P0306 together?
Look for a shared cause, such as a common wiring branch, connector, injector harness or a head gasket leak between the two cylinders.
How do I tell a bad coil from a bad plug?
Swap the coil with a neighbouring cylinder’s. If the misfire code follows the coil, the coil is bad; if not, check the plug and injector.
Is it safe to drive with P0306?
Only for short trips and with care, because a continuing misfire can damage the catalytic converter.
How much does it cost to fix P0306?
Often $40–$300 for a plug or coil, but low compression or a head gasket leak can cost much more.
