Misfire Counters

DESCRIPTION
Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred. A current and a history misfire counter is maintained for each cylinder. The misfire current counters (Misfire Cur #1 - 8) indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counters displays real time data without a misfire DTC stored. The misfire history counters (Misfire Hist #1 - 8) indicate the total number of cylinder firing events which were misfires. The misfire history counters displays 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events. The Misfire counters graphic illustrates how these misfire counters are maintained.

DTC P0300 - MISFIRE DETECTED
When crankshaft rotation is erratic, the PCM detects a misfire condition. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring. The Misfire Counters graphic shows there are misfires counted from more than one cylinder. Cylinder #1 has the majority of counted misfires. In this case, the Misfire Counters would identify cylinder #1 as the misfiring cylinder. The misfires in the other counters were just background noise caused by the erratic rotation of the crankshaft. If the number of accumulated misfires is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300 - Misfire Detected.

MONITORING MISFIRE COUNTER DATA
Use Techline equipment to monitor the misfire counter data on applicable vehicles. Knowing which specific cylinder(s) misfire can lead to the root cause. Using the information in the misfire counters, will identify which cylinder(s) are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders.

NON-EMISSION SYSTEM MALFUNCTION MISFIRES
The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:

^ Contaminated fuel
^ Running out of fuel
^ Fuel fouled spark plugs
^ Basic engine fault