OBD II Systems
The 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 120,000 or 150,000 mile emission standards. If a system or component exceeds emission thresholds or fails to operate within a manufacturer's specifications, a DTC is stored and the MIL is illuminated within 2 drive cycles.
OBD II system monitors for malfunctions either continuously, (regardless of driving mode), or non-continuously, (once per drive cycle during specific drive modes). A pending DTC is stored in the PCM keep alive memory (KAM) when a malfunction is initially detected. Pending DTCs are displayed as long as the malfunction is present. Note that OBD regulations required a complete malfunction-free monitoring cycle to occur before erasing a pending DTC. This means that a pending DTC is erased on the next power-up after a malfunction-free monitoring cycle. However, if the malfunction is still present after 2 consecutive drive cycles, the MIL is illuminated. Once the MIL is illuminated, 3 consecutive drive cycles without a malfunction detected are required to extinguish the MIL. The DTC is erased after 40 engine warm-up cycles once the 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 the KAM at the point the malfunction is initially detected and the pending DTC is stored. Freeze frame data consists of parameters such as engine RPM, engine load, vehicle speed or throttle position. Freeze frame data is updated when the malfunction is detected again on a subsequent drive cycle and a confirmed DTC is stored; however, a previously stored freeze frame is overwritten if a higher priority fuel or misfire malfunction is detected. This data is accessible with the scan tool to allow duplicating the conditions when the malfunction occurred in order to assist in repairing the vehicle.
OBD I/M readiness indicators show whether all of the OBD monitors have been completed since the last time the KAM or the PCM DTCs have been cleared. Mazda stores a DTC P1000 and blinks the MIL after 15 seconds of key-on engine-off time to indicate that some monitors have not completed. In some states, it may be necessary to carry out an OBD check in order to renew a vehicle registration. The IM readiness indicators must show that all monitors have been completed prior to the OBD check.
The following monitor descriptions provide a general description of each OBD monitor. In these descriptions, the monitor strategy, hardware, testing requirements, and methods are presented to provide an overall understanding of monitor operation. An illustration of each monitor may also be provided. These illustrations should be read as typical examples and are not intended to represent all possible vehicle configurations.
Each illustration depicts the PCM as the main focus with primary inputs and outputs for each monitor. The icons to the left of the PCM represent the inputs used by each of the monitor strategies to enable or activate the monitor. The components and subsystems to the right of the PCM represent the hardware and signals used while carrying out the tests and the systems being tested. The CCM illustration has numerous components and signals involved which are shown generically. When referring to the illustrations, match the numbers to the corresponding numbers in the monitor descriptions for a better comprehension of the monitor and associated DTCs.
These icons are used in the illustrations of the OBD monitors and throughout this service information.