Misfire Counters

Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time when the misfire occurred. The misfire counters are basically a file on each engine cylinder. A current and a history misfire counter is maintained for each cylinder. The misfire current counters, Misfire Cur #1-6, indicate the number of firing events out of the last 200 crankshaft revolutions that were misfires. The misfire current counters will display real time data without a misfire DTC being stored. The misfire history counters, Misfire History #1-6, indicate the total number of cylinder misfires. The misfire history counters will display 0 until the misfire diagnostic fails and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 crankshaft revolutions.

The misfire counters graphic illustrates how these misfire counters are maintained. If the misfire diagnostic reports a failure, the diagnostic executive reviews all of the misfire counters before reporting a DTC. This way, the diagnostic executive reports the most current information.

When crankshaft rotation is erratic, a misfire condition will be detected. 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 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.




Use the Techline equipment in order to monitor the misfire counter data on the OBD II compliant vehicles. Knowing which specific cylinders misfired can lead to the root cause, even when dealing with a multiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders number 1 and 4 misfired, for example, look for a circuit or a component common to both cylinders, such as an open ignition coil in the electronic ignition system.

Misfire counter information is located in the Specific Eng. menu, Misfire Data sub-menu of the data list. The misfire diagnostic may identify a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples of this include the following items:
^ Contaminated fuel
^ Running out of fuel
^ Fuel fouled spark plugs
^ A basic engine fault