Fault codeP0309

SEVERE

Cylinder 9 Misfire Detected

What this code means

Cylinder 9 Misfire Detected.

P0309 is a generic code: it means the same thing on every make. P0309 belongs to family P03xx — Ignition and misfire. Its page explains how to read a code and groups all its codes by part.

Combustion (misfire detection)

The control unit watches the speed of the crankshaft at every combustion. When a cylinder fails to fire, the crankshaft slows slightly: the control unit counts these misfires cylinder by cylinder.

What you notice

Shaking, jerks, loss of power; a flashing warning light means unburnt fuel is reaching the catalytic converter and destroying it.

How to check it

Swapping the spark plug or the coil of the cylinder concerned with a neighbour shows whether the fault follows the part. If it stays with the cylinder, look at the injector, then a compression test.

What to do

Have it checked promptly. A misfire sends unburnt fuel into the exhaust and destroys the catalytic converter within a few dozen kilometres. If the light is flashing, stop driving.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
0standardised code
3Ignition and misfire
09P0309