P0309 — Cylinder 8 Misfire Detected
Drivable short-term, but fix now to avoid damage.
What P0309 means
This code means the engine computer detected a misfire in cylinder 8, so that cylinder isn't burning its fuel-air mixture properly. Common culprits include worn spark plugs, a failing ignition coil, a clogged fuel injector, or low compression. Repairs often begin with new spark plugs and coils, and it's a frequent issue that's usually fixable without tearing into the engine.
Common symptoms
- Check engine light is on or flashing
- Rough idle or engine shaking
- Reduced power when accelerating
- Engine stumbles or hesitates
- Increased fuel consumption
Common causes
- Worn or fouled spark plug on cylinder 8
- Faulty ignition coil for cylinder 8
- Clogged or failing fuel injector
- Vacuum leak or low compression in the cylinder
- Damaged spark plug wires or connectors
Typical repair cost
$150 – $1200
Varies by vehicle and root cause.
Frequently asked questions
Can I keep driving with a P0309 code?
Only for a short distance if absolutely needed. A misfire can damage your catalytic converter and get worse fast, especially with a flashing light. Have it diagnosed as soon as you can.
How much does it cost to fix a P0309 code?
Repairs typically range from $150 to $1,200. Replacing a spark plug or coil is cheaper, while injector or compression problems push the cost higher.
Is a P0309 code serious?
Yes. An ongoing misfire hurts performance, wastes fuel, and risks damaging costly emissions components. A flashing check engine light makes it urgent.
Could one bad coil cause the P0309 misfire?
Absolutely. A single failing ignition coil on cylinder 8 is one of the most common causes. Many mechanics test it by swapping the coil to another cylinder to see if the misfire follows.