Under CARB/EPA regulations, a vehicle's OBD system must detect a malfunction or the deterioration of an emissions-related component or system. OBD-II required automobile manufacturers to install an on-board diagnostic system capable of:

1. Identifying component degradation or a malfunction of major emissions-related systems that could prevent a vehicle from complying with federal emissions standards.

2. Alerting the vehicle operator of the need to maintain and repair emissions-related components and systems.

3. Storing standardized Diagnostic Trouble Codes (DTC) and providing access to the vehicle's onboard information with a generic scan tool.

OBD II is a complex computer hardware system (integrated circuits & chips) and very sophisticated” software. It utilizes a network of inputs and outputs to check the status of individual components, various systems, and itself (PCM). OBD II predicts tailpipe emissions. OBD II is an excellent diagnostic tool, but it does not:

  • function the same way on each make & model.
  • monitor everything that could cause high emissions.
  • monitor all operating conditions.
  • measure actual exhaust emissions.

For light-duty vehicles (LDV) and light-duty trucks (LDT), regulations require that the OBD system monitor the performance, detect malfunctions and deterioration of the catalytic converter and oxygen sensor, and detect an engine misfire.  When the OBD system detects such a problem, a trouble code must be stored in the POWERTRAIN Control Module (PCM).