P0309 Code: Cylinder 9 Misfire Detected
🟠 Moderate severity · drivable
If the light is flashing, stop immediately. Otherwise a short drive to a shop is usually fine.
What P0309 means
Cylinder 9 is misfiring — it isn't firing properly. A specific-cylinder misfire usually points to a part on that cylinder, most often the spark plug or ignition coil.
Symptoms
- Check engine light on (flashing if severe)
- Rough idle or shaking
- Loss of power
- Poor fuel economy
Common causes
- most common: Bad spark plug on cylinder 9
- most common: Failing ignition coil or plug wire for cylinder 9
- common: Clogged or leaking fuel injector on cylinder 9
- less common: Vacuum leak affecting that cylinder
- less common: Low compression in cylinder 9
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Replace the cylinder 9 spark plug | Yes | $10–50 |
| Replace the cylinder 9 ignition coil (or swap it to confirm) | Yes | $50–200 |
| Clean or replace the cylinder 9 fuel injector | Mechanic | $100–300 |
| Run a compression test on cylinder 9 | Mechanic | $100–200 |
Frequently asked questions
What is the most common cause of P0309?
A worn spark plug or failing ignition coil on cylinder 9.
Can I drive with a P0309 code?
Short distances only if the light is steady. Stop and get a tow if the light is flashing.
How much does it cost to fix a P0309?
Usually $10–$200 for a spark plug or ignition coil.
Related codes
P0300
P0300 Code: Random/Multiple Cylinder Misfire Detected
P0308P0308 Code: Cylinder 8 Misfire Detected
P0310P0310 Code: Cylinder 10 Misfire Detected
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.