The first generation of On-Board Diagnostics (called OBD I) was developed by
the California Air Resources Board (CARB) 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 “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:
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 consist 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 be set. Below is an
example to illustrate the structure of the digits:
2. General Information
2.1 On-Board Diagnostics (OBD) II
2.2 Diagnostic Trouble Codes (DTCs)
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.
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