If you have fault codes for specific cylinder misfires, the first step is to swap the ignition coil assembly from the indicated cylinder, with the coil assembly from another cylinder. Next, clear the fault codes and drive the vehicle. If the fault code now comes up indicating the new cylinder location (the cylinder that the coil was moved to), you can assume that the coil assembly is at fault and replace the coil. If the original fault code returns (same cylinder as the original misfire code), you can assume that the fault involves this specific cylinder. The fault could be a fouled or worn spark plug, fuel injector problem, vacuum leak at this cylinder or a mechanical problem within the cylinder (leaking intake or exhaust valve, carbon build-up, etc.).

If the fault code returns to the original cylinder, after the coil swap and code clearing (as noted above), the next course of action should be to inspect the spark plug. The plug may be worn or fouled, causing the misfire. If the spark plug looks visibly fine (if not, replace it), swap it with another cylinder, similar to the coil swap. Clear the fault codes and see if the fault returns. In the same manner as the coil swap, test, if the fault code moves to the new cylinder, replace the spark plug (in fact, you should just replace them all, in this case). If the code returns on the original cylinder, further testing and diagnosis will be required in order to determine if the fault is caused by the fuel injector, a vacuum leak or an issue within the cylinder.