Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on the crankshaft rotational variations, or reference period. The control module determines the crankshaft rotational velocity using the Crankshaft Position (CKP) sensor and the Camshaft Position (CMP) sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1000-3200 engine revolutions.

For a catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road applies torque to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity. The control module detects this as a false misfire.

On the automatic transmission equipped vehicles, the Torque Converter Clutch (TCC) will disable whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs on the crankshaft rotation.

When the TCC has disabled as a result of misfire detection, the TCC will re-enable after approximately 3200 engine revolutions if no misfire is detected. The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to re-evaluate the system.

During a transmission high temperature condition, the misfire diagnostic will disable and the TCC will operate normally. This avoids further increasing the temperature of the transmission.




Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred. These misfire counters are basically a tile on each engine cylinder.

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 in 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 DTC P0300 is set. Once the misfire DTC sets, the misfire history counters are updated every 200 cylinder firing events.

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 the crankshaft rotation is erratic, the control module 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 that misfires are counted from more than one cylinder. Cylinder # 1 has the majority of the 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 misfires accumulated is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300.

Use Techline equipment to monitor the misfire counter data on OBD II compliant vehicles. Knowing which 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, look for a circuit or component common to both cylinders number 1 and 4 such as an open ignition coil in an electronic ignition system.

The misfire counter information is located in the Misfire Data menu of the data list.

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:
^ A contaminated fuel
^ Running out of fuel
^ Any fuel-fouled spark plugs
^ A basic engine fault