Description of On-Board Diagnostics

OVERVIEW

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 requires monitoring of emission-related components. The malfunction indicator lamp (MIL) is required to light and alert the driver of a malfunction and the need for service of the emission control system. A diagnostic trouble code (DTC) is associated with the MIL identifying the specific area of the fault.

NOTE: The OBD system is used on vehicles under 14,000 GVW.

The OBD 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 monitors detect system faults and initiates DTC setting and MIL activation. Fault detection strategy and MIL operation are associated with drive cycles. An OBD pending DTC is stored in the PCM keep alive random access memory (RAM) when a fault is first detected. The MIL is turned on after two consecutive drive cycles with faults. 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 operation of each of the OBD monitors is discussed in detail.

The On Board diagnostic computer program in the electronic powertrain control module (PCM) coordinates the OBD 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.

OBD inspection maintenance (IM) readiness DTC P1000 indicates that not all of the OBD monitors have been completed since the PCMs keep alive random access memory was last cleared. In some states, it may not be possible to obtain vehicle registration if P1000 is detected during inspection. To erase DTC P1000 from the PCM, operate the vehicle until the DTC is cleared using the manufacturer's specified drive cycle.

The On Board Diagnostic System is comprised of the comprehensive component monitor, glow plug monitor, misfire detection monitor and exhaust gas recirculation (EGR) monitor.