Comprehensive Components

Along with the major monitors, OBD II requires that the diagnostic system monitor any component that could affect emissions levels. In many cases, these components were being tested under OBD I. The OBD I requirements focused mainly on testing emissions-related components for electrical opens and shorts.

However, OBD II also requires that inputs from powertrain components to the PCM be tested for rationality and that outputs to powertrain components from the PCM be tested for functionality. Methods for monitoring the various Comprehensive Component monitoring include:
- Circuit Continuity
- Open
- Shorted to Voltage
- Shorted to Ground
- Rationality and Functionality
- Outputs Tested for functionality

NOTE: Comprehensive component monitors are continuous. Therefore, enabling conditions do not apply. All will set a DTC and illuminate the MIL in 1 trip.

Input Rationality - While input signals to the PCM are constantly being monitored for electrical opens and shorts, they are also tested for rationality. This means that the input signal is compared against other inputs and information to see if it makes sense under the current conditions.

PCM sensor and CAN Bus inputs that are checked for rationality include:
- Manifold Absolute Pressure (MAP) Sensor
- Oxygen Sensor (O2S) (slow response)
- Engine Coolant Temperature (ECT) Sensor
- Camshaft Position (CMP) Sensor
- Vehicle Speed from the Controller Antilock Brake (CAB)
- Crankshaft Position (CKP) Sensor
- Mass Air Flow (MAF)/Intake Air Temperature (IAT) Sensor
- Accelerator Pedal Position Sensor (APPS)
- Throttle Position Sensor (TPS)
- Knock Sensors
- Oxygen Sensor Heater
- Engine Controller
- Brake Switch
- Evaporative Vacuum Leak Detection (EVLD)
- P/N Switch
- Transmission Controls

Output Functionality - PCM outputs are tested for functionality in addition to testing for opens and shorts. When the PCM provides a voltage to an output component, it can verify that the command was carried out by monitoring specific input signals for expected changes. For example, when the PCM commands the Electronic Throttle Control (ETC) Motor to a specific position under certain operating conditions, it expects to see a specific (target) idle speed rpm. If it does not, it stores a DTC.

PCM outputs monitored for functionality include:
- Fuel Injectors
- Air Pump Switchover Solenoid
- Short Runner Valve Solenoid
- Ignition Coils
- Throttle Body (Electronic Throttle Control/Throttle Position Sensor)
- Purge Solenoid
- EGR Solenoid
- Radiator Fan Control
- Transmission Controls