The numbers below refer to the step numbers on the diagnostic table.
If the Misfire Current counters are incrementing and there is no apparent misfire, an erratic CKP sensor signal could be the cause. Perform the diagnostic table for DTC P0335 first if this condition is suspected.
If a misfire is present and you suspect a fuel control problem, force the fuel system into Open Loop using the scan tool and allow the engine to run for a few minutes. If this eliminates the misfire, refer to any fuel control related DTCs which are set. If no other DTCs are set, refer to the Engine Scan Tool Data List.
A misfire may not be apparent at idle. The misfire may only occur above idle under a load. Road test the vehicle and monitor the misfire current counters.
If more than one cylinder is misfiring, the misfire current counters may only increment for one cylinder. Example: Cylinders 1 and 8 are both misfiring, yet only cylinder 8 increments on the misfire current counter.
If one of the injector fuses is open, only two or three misfire current counters may increment for the corresponding side of the engine.
Running the vehicle out of fuel causes sufficient misfire to set DTC P0300. A vehicle that is out of fuel may have fuel level DTCs also set.
Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire. If there is a misfire in Cylinder #7 it is possible that water has collected in the fuel rail.
If there is a misfire detected in Cylinder #4or #6 it is possible that the fuel pressure regulator diaphragm has ruptured, causing fuel to be drawn in through the regulator vacuum line. Remove the vacuum line and inspect for fuel contamination.
A restricted fuel filter can cause sufficient misfire to set DTC P0300.
Excessive vibration from sources other than the engine could cause a misfire DTC. The following are possible sources of vibration:
Variable thickness brake rotor
Drive shaft not balanced
Certain rough road conditions
Observe, if more then one cylinder is misfiring, the scan tool may only display one cylinder misfiring. This will not be apparent until the repair is completed. Also, if an ignition coil ground circuit is open for one side of the engine, the scan tool may only display 2 or 3 cylinders misfiring. Inspect the ground circuit for the ignition coil on the cylinder bank of the engine that has more then one cylinder misfiring.