What causes fault code P0309?

• Most likely: Defective ignition coil (COP) or spark plug on cylinder 9 • Likely: Fuel injector circuit fault or clogged injector • Possible: Intake manifold vacuum leak near cylinder 9 runner • Rare: Mechanical engine problem (low compression, valve train issue) or wiring harness damage

How do you fix fault code P0309?

STEP 1: Inspect spark plug and ignition coil on cylinder 9. Swap the coil with another cylinder to see if the code follows (e.g., changes to P0310). STEP 2: Measure supply voltage to the coil (should be battery voltage) and check ground. Test fuel injector resistance (typically 12-16 Ohms). STEP 3: Perform a compression test or leak-down test on cylinder 9 to rule out mechanical internal engine damage. Use a smoke machine to check for vacuum leaks. STEP 4: Replace the faulty component (plug, coil, or injector) and clear the DTC.

Powertrain (Engine & Gearbox)Standard EOBD

P0309 - Cylinder 9 Misfire Detected

Q: What are the symptoms of fault code P0309?

• Check Engine light is on or flashing • Engine hesitation or jerking while accelerating • Rough idling and engine vibration • Noticeable loss of power • Fuel smell from the exhaust and decreased fuel economy

Last updated:1 February 2026

Related:Spark plug, Ignition Coil, Fuel Injector, Crankshaft Position Sensor, Engine Control Unit (ECU)

System

Powertrain (Engine & Gearbox)

Related Fault Codes

P0300

Random or Multiple Cylinder Misfire Detected

P0301

Cylinder 1 Misfire Detected

P0302

Cylinder 2 Misfire Detected

P0303

Cylinder 3 Misfire Detected

Technical Description

P0309 is an OBD-II diagnostic trouble code (DTC) indicating that the Engine Control Module (ECM) has detected a misfire on cylinder number 9. This is typically monitored via the Crankshaft Position (CKP) sensor measuring subtle fluctuations in rotational velocity. If cylinder 9 fails to fire properly, the crankshaft speed drops during its power stroke. A flashing MIL (Check Engine Light) indicates a severe misfire that could lead to catalytic converter damage due to unburned fuel entering the exhaust stream.

Possible Causes

Defective ignition coil (COP) or spark plug on cylinder 9

Fuel injector circuit fault or clogged injector

Intake manifold vacuum leak near cylinder 9 runner

Mechanical engine problem (low compression, valve train issue) or wiring harness damage

Symptoms

Check Engine light is on or flashing

Engine hesitation or jerking while accelerating

Rough idling and engine vibration

Noticeable loss of power

Fuel smell from the exhaust and decreased fuel economy

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P0309 - Cylinder 9 Misfire Detected

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P0309 - Cylinder 9 Misfire Detected

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