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Misfire Counters

Whenever a cylinder misfires, misfire diagnostic counts misfires and notes crankshaft position at the time misfire occurred. These "misfire counters" are basically a file on each cylinder. A current and history misfire counter are maintained for each cylinder. Current misfire counters (MISFIRE CUR #1-6) indicate number of firing events out of the last 100 cylinder firing events which were misfires. Current misfire counter will display real time data without a misfire DTC stored. History misfire counters (MISFIRE HIST #1-6) indicate total number of cylinder firing events which were misfires. History misfire counters will display "0" until misfire diagnostic has failed and a DTC P0300 is set. Once misfire DTC P0300 is set, history misfire counters will be updated every 100 cylinder firing events. A misfire counter is maintained for each cylinder. If misfire diagnostic reports a failure, diagnostic executive reviews all misfire counters before reporting DTC. This way diagnostic executive reports most current information. When crankshaft rotation is erratic, a misfire condition will be detected. Because of this erratic condition, data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring. Misfires are counted for more than one cylinder. If cylinder No. 1 has the majority of counted misfires. In this case, Misfire counters would identify cylinder No. 1 as misfiring cylinder. Misfires in the other counters were just background noise caused by erratic misfire 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. Use diagnostic equipment to monitor misfire counter data on OBD II-compliant vehicles. Knowing which specific cylinder(s) misfired can lead to root cause, even when dealing with a multiple cylinder misfire. Using information in misfire counters identifies which cylinders are misfiring. If misfire counter indicates cylinders 1 and 4 misfired, look for a circuit or component common to both cylinders. On scan tool, misfire counter information is located in the "Eng". menu, "Misfire Data" sub-menu of the data list. Misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include following items:

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