P0309 - Cylinder 9 Misfire Detected

Cylinder 9 is misfiring often enough for the ECM to detect it.

Code Overview

Cylinder 9 is misfiring often enough for the ECM to detect it.

Common Causes

• Worn or fouled spark plug on cylinder 9
• Weak ignition coil, boot, or plug wire for cylinder 9
• Fuel injector issue on cylinder 9
• Vacuum leak affecting the cylinder 9 runner
• Low compression or valve sealing problem on cylinder 9

Symptoms

• Rough idle or steady shake
• Hesitation or stumble under load
• Fuel smell or flashing MIL if the misfire is severe

Diagnostic Steps

1. Swap the coil or plug with another cylinder and see whether the misfire follows
2. Inspect the cylinder 9 spark plug, boot, and plug well for wear or oil intrusion
3. Check injector operation and connector condition on cylinder 9
4. Perform compression or leak-down testing if the misfire stays on the same hole

Repair Difficulty

Moderate

General difficulty estimate for the most common repair path.

Likely Repairs & Cost Guides

Use test results to confirm the fault before pricing the repair.

Related OBD Codes

Browse the closest code-family pages when the same fault pattern often shows up nearby.

Next Steps

Move from the code to a diagnosis path, then into repair pricing once the fault is confirmed.