Introduction to On Board Diagnostics (II)

Functionality of PowertrainControl Module (PCM) inOBD II OBD IIIf the fault persists for two cycles consecutively, it is not erased until the defectis repaired by the technician. In case of an out of range output of a sensor , thePCM substitutes a corresponding reasonable value for that vehicle conditionof operation. The PCM also provides clear information to the driver byilluminating the MIL (Malfunction Indicator Light) in case of a defect withoutcausing alarm for minor problems. All relevant data for off-board diagnostics,and repair are stored by the PCM in its memory for later use. In the case of adefect that completely impairs the vehicle performance the PCM has the fullcapability to switch the vehicle state to a safe state of lesser capability called“Limp Home” state, in which the vehicle is brought to a safe degraded operatingcondition, that includes a halt of the vehicle. The PCM communicates with theOBD II scan tool and provides diagnostic data, and OBD II DTCs of all faultsexperienced by the vehicle so far to the external tester to facilitate off-boarddiagnostics, and vehicle repair. In this respect OBD II provides SAE J 1850 datalink for communication of diagnostic data, SAE J 2012 provides the DTC messageformats, and SAE J 1979 provides the test modes, requesting PCM for emissionsrelated powertrain diagnostics data.