Onboard Diagnostics
DIAGNOSTIC TROUBLE CODES (DTCs)
¾ To expand emissions-related system monitoring. This includes a set of
computer run diagnostics called Monitors. Monitors perform diagnostics
and testing to verify that all emissions-related components and/or systems
are operating correctly and within the manufacturer’s specifications.
¾ To use a standardized Diagnostic Link Connector (DLC) in all vehicles.
(Before OBD 2, DLCs were of different shapes and sizes.)
¾ To standardize the code numbers, code definitions and language used to
describe faults. (Before OBD 2, each vehicle manufacturer used their own
code numbers, code definitions and language to describe the same faults.)
¾ To expand the operation of the Malfunction Indicator Lamp (MIL).
¾ To standardize communication procedures and protocols between the
diagnostic equipment (Scan Tools, CanScans, etc.) and the vehicle’s
on-board computer.
OBD 2 Terminology
The following terms and their definitions are related to OBD 2 systems. Read
and reference this list as needed to aid in the understanding of OBD 2
systems.
¾ Powertrain Control Module (PCM) - The PCM is the OBD 2 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.).
¾ 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 eleven Monitors are used in OBD 2 systems.
Additional Monitors will be added as the OBD 2 system is further
developed.
Not all vehicles support all eleven Monitors.
D900 CanScan 16
¾ 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
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