P0309 Cylinder 9 Misfire Detected

P0309 indicates a misfire detected on cylinder 9. The cylinder is not burning consistently enough to match expected crankshaft speed changes.

Code details

Generic OBD-II · P0

System: Ignition / Misfire
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not keep driving with an active cylinder 9 misfire because catalyst damage can follow.

Symptoms of P0309

Rough idle, shaking, or a stumble under acceleration.
A flashing check-engine light during an active misfire.
Power loss and higher fuel consumption.

Common causes of P0309

Worn spark plugs, ignition coils, or plug wires.
Fuel-delivery problems — injectors, pressure, or a lean/rich mixture.
Vacuum leaks, low compression, or a mechanical fault in that cylinder.

Useful checks before replacing parts

Compare cylinder 9 misfire counters, freeze-frame load, fuel trims, ignition command, injector command, and compression data.
Inspect spark or glow components where applicable, injector operation, vacuum leaks, wiring, compression, fuel quality, and exhaust restriction.
Check whether the misfire is cold-only, hot-only, idle-only, load-dependent, or paired with injector circuit or fuel trim codes.

Related OBD-II codes

These nearby ignition / misfire entries can help compare wording and pin down the fault area.