OBD2 Terminology & Reference Table

OBD2 Terminology

The following terms and their definitions are related to OBD2 systems. Read and reference this list as needed to aid in the understanding of OBD2 systems.

Powertrain Control Module (PCM)

The PCM is the OBD2 accepted term for the vehicle’s Onboard computer. In addition to controlling the engine management and emissions systems, the PCM also participates in controlling the powertrain (transmission) operation. Most PCMs also have the ability to communicate with other computers on the vehicle (ABS, ride control, body, etc.).

Monitor

Monitors are Diagnostic Routines programmed into the PCM. The PCM utilizes these programs to run diagnostic tests and to monitor the operation of the vehicle’s emissions-related components or systems to ensure they are operating correctly and within the vehicle’s manufacturer specifications. Currently, up to fifteen monitors are used in OBD2 systems. Additional monitors will be added as the OBD2 system is further developed.

Enabling Criteria

Each Monitor is designed to test and monitor the operation of a specific part of the vehicle’s emissions system (EGR system, oxygen sensor, catalytic converter, etc.). A specific set of Conditions or Drive Cycle Procedures must be met before the computer can command a monitor to run tests on its related system. These Conditions are known as Enabling Criteria. The requirements and procedures vary for each Monitor. Some Monitors only require the ignition key to be turned ON for them to run and complete their diagnostic testing. Others may require a set of complex procedures, such as starting the vehicle when cold, bringing it to operating temperature, and driving the vehicle under specific conditions before the Monitor can run and complete its diagnostic testing.

Complete/Incomplete

The terms Complete or Incomplete are used throughout this manual. Complete, means the PCM has commanded a particular monitor to perform the required diagnostic testing on a system to ensure the system is operating correctly (within factory specifications). The term Incomplete means the PCM has not yet commanded a particular monitor to perform diagnostic testing on its associated part of the emissions system.

Trip

A Trip for a particular monitor requires that the vehicle is being driven in such a way that all the required Enabling Criteria for the monitor to run and complete its diagnostic testing are met. The Trip Cycle for a particular monitor begins when the ignition key is turned ON. It's successfully completed when all the Enabling Criteria for the monitor to run and complete its diagnostic testing are met by the time the ignition key is turned OFF. Since each of the fifteen monitors is designed to run diagnostics and testing on a different part of the engine or emissions system, the Trip Drive Cycle needed for each individual monitor to run and complete varies.

OBD2 Drive Cycle

An OBD2 Drive Cycle is an extended set of driving procedures that takes into consideration the various types of driving conditions encountered in real life. These conditions may include starting the vehicle when it is cold, driving the vehicle at a steady speed (cruising), accelerating, etc. An OBD2 Drive Cycle begins when the ignition key is turned ON (when cold) and ends when the vehicle has been driven in such a way as to have all the Enabling Criteria met for all its applicable monitors. Only those trips that provide the Enabling Criteria for all monitors applicable to the vehicle to run and complete their individual diagnostic tests qualify as an OBD2 Drive Cycle. OBD2 Drive Cycle requirements vary from one model of vehicle to another. Vehicle manufacturers set these procedures. Consult your vehicle’s service manual for OBD2 Drive Cycle procedures.

Warm-up Cycle

Vehicle operation after an engine-off period where engine temperature rises at least 40°F (22°C) from its temperature before starting, and reaches at least 160°F (70°C). The PCM uses Warm-Up Cycles as a counter to automatically erase a specific code and related data from its memory. When no faults related to the original problem are detected within a specified number of Warm-Up Cycles, the code is erased automatically.

OBD2 Reference Table

The table below lists current OBD2 Monitors and indicates the following for each Monitor:

A. Monitor Type (how often does the Monitor run; Continuous or Once per trip)

B. Number of trips needed, with a fault present, to set a pending DTC

C. Number of consecutive trips needed, with a fault present, to command the MIL “On” and store a DTC

D. Number of trips needed, with no faults present, to erase a Pending DTC

E. Number and type of trips or drive cycles needed, with no faults present, to turn off the MIL

F. Number of warm-up periods needed to erase the DTC from the computer’s memory after the MIL is turned off