Misfire Monitoring

REQUIREMENTS
An OBD II system must be able to detect and react to two different misfire situations. The difference lies in the severity of the fault. If the problem is mild we react accordingly, a severe problem demands a more rapid and forceful response. The two types of misfire are:

Catalyst Damaging Misfire
A level of misfire so severe that there is danger of damaging the catalyst.

Emissions Related Misfire
Misfire which will result in the vehicle failing an inspection/maintenance (I/M) test or which causes emissions to exceed the federal test procedure (FTP) standards by 50%.

Regulations require misfire monitoring under all positive speed and load conditions - (defined as cruise, acceleration and idle). The diagnostic procedure must discriminate between single versus multiple (random) cylinder misfiring. If a single cylinder misfires, the diagnostic must identify the specific cylinder. The conditions under which the misfire occurs must be identified, even if this requires overwriting freeze frame data stored for a less critical fault.