Onboard Diagnostics
OBD2 TERMINOLOGY
7
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 “on-board 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 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.
Not all vehicles support all fifteen Monitors.
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 “driving 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.
Monitor Has/Has Not Run - The terms “Monitor has run” or
“Monitor has not run” are used throughout this manual. “Monitor
has run,” 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
“Monitor has not run” 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 Drive Cycle” for a particular Monitor begins when the
ignition key is turned “On.” It is 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.
