Equus 3110 - CanOBD2 Diagnostic Tool - Manual (Page 20)

18 CAN OBD2
E
Onboard Diagnostics
COMPUTER ENGINE CONTROLS
■ To use a standardized Diagnostic Link Connector (DLC) in all vehi-
cles. (Before OBD 2, DLCs were of different shapes and sizes.)
■ 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.)
■ To expand the operation of the Malfunction Indicator Lamp (MIL).
■ To standardize communication procedures and protocols between
the diagnostic equipment (Scan Tools, Code Readers, 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.
■ 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.).
■ 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.
■ 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 sys-
tem. 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 com-
plex 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.