Autel AutoLink AL539B [4/92] General information

2. General Information

2.1 On-Board Diagnostics (OBD) II

The first generation of On-Board Diagnostics (called OBD I) was

developed by the California Air Resources Board (ARB) and

implemented in 1988 to monitor some of the emission control

components on vehicles. As technology evolved and the desire to

improve the On-Board Diagnostic system increased, a new generation

of On-Board Diagnostic system was developed. This second generation

of On-Board Diagnostic regulations is called "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 specific components and vehicle conditions. When a

problem is detected, the OBD II system turns on a warning lamp (MIL)

on the vehicle instrument panel to alert the driver typically by the

phrase of “Check Engine” or “Service Engine Soon”. The system will

also store important information about the detected malfunction so

that a technician can accurately find and fix the problem. Here below

follow three pieces of such valuable information:

1) Whether the Malfunction Indicator Light (MIL) is

commanded 'on' or 'off';

2) Which, if any, Diagnostic Trouble Codes (DTCs) are stored;

3) Readiness Monitor status.

2.2 Diagnostic Trouble Codes (DTCs)

OBD II Diagnostic Trouble Codes are codes that are stored by the

on-board computer diagnostic system in response to a problem found

in the vehicle. These codes identify a particular problem area and are

intended to provide you with a guide as to where a fault might be

occurring within a vehicle. OBD II Diagnostic Trouble Codes

consists of a five-digit alphanumeric code. The first character, a letter,

identifies which control system sets the code. The other four

characters, all numbers, provide additional information on where the

DTC originated and the operating conditions that caused it to set. Here

below is an example to illustrate the structure of the digits: