Check Engine Light Code P0309
Misfire Detection Monitor - Cylinder 9 Misfire
How does a P0309 code trigger the check engine light? This code will trigger the check engine light as follows:
The misfire detection monitor, a software strategy built into the computer, is designed to detect an engine misfire. The computer can also normally identify the specific cylinder in which the misfire has occurred. A misfire is nothing more than a lack of combustion, which can be caused by poor fuel quality or metering, low compression, lack of spark or unmetered air entering the engine. There are other possible, less obvious causes as well, such as uncommanded Exhaust Gas Recirculation (EGR), flow.
Fuel injectors, related wiring, sensors and computer issues
Running out of gas, or poor fuel quality
Evaporative emissions system (EVAP) concerns: fuel vapors leaking into engine
Incorrect Fuel Pressure
EGR system concerns: leaking EGR valve or restricted ports
Base engine concerns: low compression, valve train problems and timing issues
Ignition system concerns including, but not limited to:
Faulty spark plugs
Faulty coil or related wiring
Ignition module or related wiring issues
Ignition related sensor faults or wiring issues
To diagnose a P0309 trouble code we begin by checking for proper fuel and ignition system operation, and then follow up with the less likely causes listed above, such as EGR and EVAP system problems. New style coil on plug applications have a high failure rate, and can be concluded faulty by swapping to another cylinder and checking to see if the misfire moves to that cylinder. This is a quick check if a capable scan tool or oscilloscope is not available. Always make sure the basic maintenance is done first and that things such as the fuel filter, air filter, spark plugs and spark plug wires are in good working order, as these are all possible candidates for a problem. If necessary, check for wiring and component concerns. See our article Automotive Circuit Testing 101, if you need more assistance with this. If concern is determined to be intermittent, check out our article on intermittent diagnosis and wiggle test connectors and wiring, attempting to duplicate concern. You may also use the "Get Help" link if you need specifications or have any other related questions. Remember to refer to an appropriate manual for specific instruction.