General System Description

The California Air Resources Board (CARB) began regulation of On Board Diagnostic (OBD) for diesel vehicles sold in California beginning with the 1997 model year OBD II requires monitoring of emission-related components. The Malfunction Indicator Lamp (MIL) was required to light and alert the driver of the malfunction and the need for service of the emission control system. The MIL must be labeled CHECK ENGINE. A fault code or Diagnostic Trouble Code (DTC) is associated with the MIL identifying the specific area of the fault.

The OBD II system meets government regulations by monitoring the emission control system. When a system or component exceeds emission thresholds or a component operates outside of tolerance, a DTC will be stored and the MIL will be illuminated.

The OBD II Monitors detect system faults and initiate DTC setting and MIL activation. Fault detection strategy and MIL operation are associated with drive cycles. An OBD II pending DTC is stored in the PCM keep alive random access memory when a fault is first detected. In most cases the MIL is turned on after two consecutive drive cycles with the fault and the DTC is set. The DTC is cleared after 40 engine warm-up cycles without the fault being detected once the MIL is turned off. Once a Monitor turns on the MIL, it will require three consecutive drive cycles without a fault for the MIL to turn off.

The On Board diagnostic computer program in the Electronic Engine Control (EEC) Powertrain Control Module (PCM) coordinates the OBD II self-monitoring system. This program controls all the monitors and interactions, DTC and MIL operation, Freeze Frame data and scan tool interface.

Freeze Frame data describes stored engine conditions such as state of the engine rpm and load at the point the first fault is detected. This data is accessible with the scan tool to assist in repairing the vehicle.

The On Board Diagnostic System is comprised of the Comprehensive Component Monitor and the Glow Plug Monitor.