OBD Monitor Operation

OBD MONITOR OPERATION

Some OBD monitors are continuous, that is, they operate all the time the ignition is on.

Some OBD monitors have conditions that must be satisfied before the monitor is allowed to operate, for example: engine speed 1000 - 4000 rpm; engine airflow 10 - 100 g/sec; intake air temperature -10 degree C - +50 degree C. These conditions ensure the vehicle is operating in such a manner that the failure may be correctly diagnosed. If the conditions are not met, it may be possible for a failure to be present on the vehicle but remain undetected by the module.

Most OBD monitors use 2-trip detection: on the first occasion the failure is detected, a pending DTC is recorded. If on the subsequent drive cycle, the failure is again detected, then a confirmed DTC is logged and the MIL may be illuminated.

A few OBD monitors operate on a single trip basis where the MIL is illuminated as soon as the failure is diagnosed.

Detection of a failure may inhibit the operation of other OBD monitors to ensure that multiple DTCs are not logged due to a single fault being present.

NOTE: The system is not infallible and a single fault may result in two or more failures being detected and hence the recording of two or more DTCs.