P0309 - Cylinder 9 Misfire Detected

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

Treat the code as the starting point: compare symptoms, likely causes, and repair paths before pricing parts or moving into an estimate.

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 symptoms, scan data, and quick checks to confirm the likely repair path before pricing parts. The estimator helps compare repair paths before replacing anything unnecessarily.

Related OBD Codes

Browse nearby code-family pages when the same fault pattern overlaps across systems, such as misfire, lean fuel trim, EVAP sealing, cooling, or charging faults.

Next Steps

Move from code lookup to diagnosis, then estimate the likely repair only after symptoms, causes, and checks point to the same path.