On Board Diagnostics (OBD) Monitors: Overview

The California Air Resources Board (CARB) began regulation of 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 concern and the need for repair of the emission control system. A diagnostic trouble code (DTC) is associated with the MIL identifying the specific area of the concern.

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 is stored and the MIL is illuminated.

The OBD monitors detect system concerns and initiate DTC setting and MIL activation. Concern detection strategy and MIL operation are associated with drive cycles. An OBD pending DTC is stored in the powertrain control module (PCM) keep alive memory (KAM) when a concern is first detected. The MIL is turned on after 2 consecutive drive cycles with concerns. The DTC is cleared after 40 engine warm-up cycles without the concern being detected once the MIL is turned off. Once a monitor turns on the MIL, it requires 3 consecutive drive cycles without a concern for the MIL to turn off. The operation of each of the OBD monitors is discussed in detail within this section.

The OBD computer program in the 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 the state of the engine RPM and load at the point the first concern 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 PCM KAM was last cleared. In some states, it may not be possible to obtain vehicle registration if DTC 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 OBD system is comprised of the comprehensive component monitor (CCM), the glow plug monitor, the misfire detection monitor, and the exhaust gas recirculation (EGR) monitor.

Starting with the 2007 model year, vehicles not required to comply with current OBD requirements use an EMD system. EMD systems are used on all California and Federal gas and diesel on-road heavy duty engines used in vehicles over 6,350 kg (14,000 lb) GVWR. EMD systems are required to functionally monitor the fuel delivery system, EGR system, particulate matter trap, as well as emission related PCM inputs for circuit continuity and rationality, and emission related outputs for circuit continuity and functionality. EMD requirements are very similar to previous OBD system requirements. As such, previous OBD system principles are employed, with the addition of some CCM rationality and functionality checks. EMD vehicles use the same PCM, controller area network (CAN), data link connector (DLC) and PCM software as the corresponding OBD vehicles.