P0309 — Cylinder 9 Misfire Detected

Diagnostic algorithm

1. Verify code: Clear codes and perform a test drive to confirm P0309 returns. Note misfire conditions (idle, load, cold).
2. Retrieve live data: monitor misfire counters, fuel trims, fuel rail pressure, and sensor data while reproducing misfire.
3. Visual inspection: check spark plug, coil, injector wiring and connectors for cylinder 9. Repair any damaged connectors or pin corrosion.
4. Ignition swap test: swap the coil (or coil pack lead) from cylinder 9 to another cylinder. If misfire follows the coil, replace the coil.
5. Injector test: listen for injector pulse, perform a back-probe to verify injector pulse, and consider swapping injectors or performing an injector flow/balance test.
6. Compression/leak-down: perform a compression or leak-down test on cylinder 9 and compare to adjacent cylinders. If low, proceed with mechanical diagnosis (valve, piston rings, head gasket).
7. Fuel and air checks: confirm correct fuel pressure and check for vacuum leaks near cylinder 9. Repair as needed.
8. Spark plug check/replace: inspect plug for fouling or damage; replace plug if out of spec and retest.
9. Check timing and sensors: verify cam/crank timing and inspect cam/crank position sensors and timing components for issues.
10. After repairs: clear codes and confirm repair with road test and re-scan. If intermittent, perform extended monitoring or cabin/drive cycle tests.
11. Safety note: When testing ignition circuits, use insulated tools and follow manufacturer safety procedures. When performing compression or leak-down tests, follow engine-specific procedures.