P0309
Cylinder 9 Misfire Detected
Ignition / misfire
Generic OBD-II
High
What it means
P0309 means the engine computer has detected a misfire on cylinder 9 — the fuel-air mix in that cylinder isn't burning properly, so the engine runs rough and loses a little power. It's most often a worn spark plug or coil, but a persistent misfire can overheat the catalytic converter, so it's worth fixing promptly.
High
Can I keep driving? Drive as little as possible — a single-cylinder misfire dumps unburnt fuel into the exhaust and can quickly damage the catalytic converter, especially if the light is flashing.
Common symptoms
What you'll notice
- Check-engine light on (may flash during an active misfire)
- Rough idle, shaking, or stumbling
- Hesitation and reduced power on acceleration
- Increased fuel consumption
Common causes
Most likely first
- Worn spark plug or failing ignition coil on cylinder 9Likely
- Clogged or faulty cylinder 9 fuel injectorCommon
- Vacuum leak affecting that cylinderCommon
- Low fuel pressureLess often
- Low compression on cylinder 9 (valve or ring wear)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data to confirm the misfire is on cylinder 9.
- Inspect and replace the cylinder 9 spark plug and coil first — the most common cause.
- Swap the coil (and plug) with an adjacent cylinder; if the misfire follows, you've found it.
- Check the injector and look for a vacuum leak near that cylinder.
- If it persists, run a compression test on cylinder 9, then clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0309?+
No, not for long. A steady misfire, especially with a flashing light, can overheat and ruin the catalytic converter within a short drive.
Can I just clear P0309?+
It will return within a few drive cycles until the cause of the cylinder 9 misfire is repaired.
Related codes
Often seen with P0309
Read & clear P0309 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX