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1 General Information
On-Board-Diagnostics (OBD) II
The first generation of On-Board Diagnostic (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 OBD I system increased,
a new generation of On-Board Diagnostics 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:
Whether the Malfunction Indicator Light (MIL) is commanded ‘on’ or ‘off’;
Which, if any, Diagnostic Trouble Codes (DTCs) are stored;
Readiness Monitor status.
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 consist of a five-digit alphanumeric code.
The first character, a letter, identifies the control system which 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:
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