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1.2 On-Board Diagnostics (OBD) II
The OBD II system is designed to monitor emission control systems and key
engine components by performing either continuous or periodic tests of specic
components and vehicle conditions, which will oer three pieces of such valuable
information:
• Whether the Malfunction Indicator Light (MIL) is commanded “on” or “o”;
• Which, if any, Diagnostic Trouble Codes (DTCs) are stored;
• Readiness Monitor status.
1.3 OBD II Denitions
The following terms and their denitions are related to OBD II systems. Read
and reference this list as needed to aid in the understanding of OBD II systems.
EOBD - European On-Board Diagnostics. Essentially the same as OBD II, with
the same Data Link Connector and Communication Protocols.
Communication Protocols - Allows dierent systems and sensors in a vehicle to
communicate. There are currently ve protocols:
• CAN Bus
• J1850 VPW
• ISO 9141-2
• J1850 PWM
• ISO 14230 KWP
PCM -- Powertrain Control Module
.
The PCM is the OBD II 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.).
DLC -- Data Link Connector. The 16-cavity connector on the vehicle that allows
communication between the computer system and the diagnostic tool.
MIL -- Malfunction Indicator Light. The vehicle’s “Check Engine” warning light
that activates when a DTC is stored.
DTC -- Diagnostic Trouble Code. A code stored in the computer system’s
memory, which helps to identify the fault condition that is causing the MIL to
activate.
Freeze Frame Data -- Operating conditions that are stored when a DTC is
stored.
PID -- Parameter Identification Data. Data returned by the vehicle’s control
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