Applicable Systems
OBD-II Universal
Symptoms
The engine control module has detected a cylinder misfire. The check engine light flashes and the engine runs rough with reduced power.
Possible Causes
- Spark plug fouled, worn, or with incorrect gap
- Ignition coil failure on the misfiring cylinder
- Fuel injector clogged or failing to deliver fuel
- Vacuum leak at the intake manifold gasket or a hose
Troubleshooting Steps
- Step 1: Connect an OBD-II scanner and read the misfire code (P0301-P0312)
The last digit identifies the misfiring cylinder number - Step 2: Swap the ignition coil from the misfiring cylinder to a known-good cylinder
If the misfire follows the coil, replace the coil - Step 3: Remove and inspect the spark plug for fouling, wear, and gap
Replace if worn; set the gap to the specification in the service manual - Step 4: If the ignition system is good, perform a compression test on the misfiring cylinder
Low compression indicates valve, piston ring, or head gasket issues
How to Reset
Reset: Clear the code with the OBD-II scanner after the repair. The check engine light turns off after 3 consecutive drive cycles without a misfire.
Common Replacement Parts
- Spark plug: Iridium or platinum, pre-gapped (Match to engine specification)
- Ignition coil: Coil-on-plug or dual-post (Match to engine model)
- OBD-II scanner: Compatible with the vehicle protocol (For code reading and clearing)
Frequently Asked Questions
- Q: What is a misfire?
Incomplete or absent combustion in one or more cylinders. Causes include ignition, fuel, compression, or timing problems. Persistent misfire can damage the catalytic converter.
- Q: How do I identify the misfiring cylinder?
Read the DTC. P0301 = cylinder 1, P0302 = cylinder 2, etc. P0300 = random/multiple cylinder misfire.
- Q: Can I drive with a misfire?
Briefly to reach a service area. Prolonged driving with a misfire can overheat and damage the catalytic converter, leading to expensive repairs.
Source: SAE J2012 Standard
Category: OBD-II