What does it mean?
This diagnostic code (DTC) is a generic powertrain code, meaning it applies to vehicles equipped with OBD-II. Although generic, specific repair steps may vary by make/model.
A P0309 code means the car’s computer has detected that one of the engine’s cylinders is not functioning properly. In this case, it’s cylinder #9.
Symptoms
Symptoms may include:
The engine may be harder to start
The engine may stumble/misfire and/or hesitate
Other symptoms may also be present
Causes
A P0309 code can mean one or more of the following has occurred:
Faulty spark plug or wire
Faulty ignition coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burnt exhaust valve
Faulty catalytic converter(s)
Fuel shortage
Poor compression
Faulty computer
Possible Solutions
If there are no symptoms, the simplest thing to do is reset the code and see if it returns.
If there are symptoms such as engine stumbling or hesitation, check all cables and connectors leading to the cylinders (i.e., spark plugs). Depending on the lifespan of the ignition components in the car, it may be wise to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap and rotor (if applicable). Otherwise, check the coils (e.g., coil packs). In some cases, the catalytic converter has failed. If you smell rotten eggs in the exhaust, your catalytic converter needs to be replaced. I’ve also heard in other cases that the issues were faulty fuel injectors.