Misfire Counters

Misfire Counters (Typical):

Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes in which cylinder it occurred. These "misfire counters" are basically a file on each engine cylinder. A misfire counter is maintained for each cylinder.
The misfire image 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 there are misfires counted from almost every cylinder. Cylinder #1 has the majority of counted misfires. In this case, the Diagnostic Executive would identify cylinder #1 as faulty and set DTC P0301. The misfires in the other counters were just background noise caused by the erratic rotation of the crankshaft. If the number of accumulated is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300 - Random / Multiple Misfire.

Use Techline equipment to monitor misfire counter data on On Board Diagnostics II (OBD II) compliant vehicles. Knowing which specific cylinder(s) 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).

Misfire counter information is located in the Specific ENG menu, Misfire Data sub-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:

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