Diagnostic Monitors

Starting with 2002 model year, all California passenger cars and trucks (up to 14,000 lbs. GVW) and all federal passenger cars and trucks (up to 8500 lbs. GVW) are required to comply with either CARB-OBD-II or EPA OBD requirements. Federal heavy-duty truck up to 10,000 lbs. GVWR choosing to certify using Light Duty Truck provisions must comply with OBD-II requirements. Federal heavy-duty trucks over 8500 lbs. GVW are not required to comply with any OBD regulation, however in order to meet minimum serviceability requirements, must comply with OBD-I requirements. OBD-II requirements apply to gasoline vehicles, diesel vehicles, ethanol flexible fuel vehicles and bi-fuel CNG/LPG vehicles while running on gasoline. OBD-II requirements are being phased in on dedicated NGVs and bi-fuel CNG/LPG vehicles while running on gaseous fuels. Passenger cars and trucks sold in Canada and Mexico have Federal calibrations, unless unique calibrations are certified for Mexico at high altitude.

OBD-II system monitors virtually all emission control systems and components that can affect tailpipe or evaporative emissions. In most cases, malfunctions must be detected before emissions exceed 1.5 times the applicable 50-100 k/mile emission standards. If a system or component exceeds emission thresholds or fails to operate within a manufacturer's specifications, a DTC will be stored and MIL will be illuminated within 2 driving cycles. OBD-II system monitors for malfunctions either continuously, regardless of driving mode, or non-continuous, once per drive cycle during specific drive modes. A pending DTC is stored in PCM Keep Alive Memory (KAM) when a malfunction is initially detected. This pending DTC may be erased on the third vehicle restart after 2 consecutive drives cycles with no malfunction. However, if malfunction is still present after 2 consecutive drive cycles, MIL is illuminated. Once MIL is illuminated, 3 consecutive drive cycles without a malfunction detected are required to extinguish MIL. DTC is erased after 40 engine warm-up cycles once MIL is extinguished.

In addition to specifying and standardizing much of the diagnostics and MIL operation, OBD-II requires the use of a standard Diagnostic Link Connector (DLC), standard communication links and messages, standardized DTCs and terminology. Examples of standard diagnostic information are freeze frame data and Inspection Maintenance (IM) readiness indicators. Freeze frame data describes data stored in KAM at the point the malfunction is initially detected. Freeze frame data consists of parameters such as engine RPM and load, state of fuel control, spark, and warm-up status. Freeze frame data is stored at the time the first malfunction is detected, however, previously stored conditions will be replaced if a fuel or misfire fault is detected. This data is accessible with scan tool to assist in repairing vehicle. OBD IM readiness indicators show whether all of the OBD monitors have been completed since last time KAM or PCM DTC(s) have been cleared. A DTC P1000 is also stored to indicate that some monitors have not completed. In some states, it may be necessary to perform an OBD check in order to renew a vehicle registration. The IM readiness indicators must show that all monitors have been completed prior to OBD check. DIAGNOSTIC MONITORING TESTS provides a general description of each OBD-II monitor. In these descriptions, monitor strategy, hardware, testing requirements and methods are presented to provide an overall understanding of monitor operation. For a description of diagnostic monitors, see DIAGNOSTIC MONITORS in SELF-DIAGNOSTICS - CNG, FLEX-FUEL & GASOLINE article.