OBD II regulations require that all emission related circuits and components controlled by the PCM that could affect emissions are monitored for circuit continuity and out-of-range faults. The Comprehensive Component Monitor (CCM) consists of 4 different monitoring strategies: 2 for inputs and 2 for output signals.
One input strategy is used to check devices with analog inputs for opens, shorts, or out-of-range values. The CCM accomplishes this task by monitoring A/D converter input voltages of various Sensors.
A second input strategy is used to check devices with digital and frequency inputs by performing rationality checks. The PCM uses other Sensor readings and calculations to determine if a Sensor or switch reading is correct under existing conditions. Some tests of the CKP, CMP and VSS run continuously. Other tests run only after actuation.
An Output Device Monitor in the PCM checks for any open and shorted circuits by observing the control voltage level of a particular device.
The control voltage is low when the device is on and the voltage is high when the device is off. Monitored outputs include the AC relay, ASD relay SS1, SS2, SS3, EGR and EVAP and O2 heater control.