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:
1. Circuit Continuity
- Open
- Shorted high
- Shorted to ground
2. Rationality or Proper Functioning
- Inputs tested for rationality
- Outputs tested for functionality

NOTE: Comprehensive component monitors are continuous. Therefore, enabling conditions do not apply.

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 inputs that are checked for rationality include:
- Manifold Absolute Pressure (MAP) Sensor
- Oxygen Sensor (O2S)
- Engine Coolant Temperature (ECT) Sensor
- Camshaft Position (CMP) Sensor
- Vehicle Speed Sensor
- Crankshaft Position (CKP) Sensor
- Intake/Inlet Air Temperature (IAT) Sensor
- Throttle Position (TPS) Sensor
- Ambient Temperature Sensors
- Power Steering Switch
- Oxygen Sensor Heater
- Brake Switch
- Leak Detection Pump Switch or NVLD switch (if equipped)
- P/N Switch

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 Idle Air Control (IAC) 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
- Ignition Coils
- Idle Air Control
- Purge Solenoid
- EGR Solenoid (if equipped)
- LDP Solenoid or NVLD solenoid (if equipped)
- Radiator Fan Control
- Trans Controls