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GENERAL INFORMATION
The rst 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 the technology evolved and the desire to improve the On-Board
Diagnostic system increased, a new generation of On-Board Diagnostic systems was developed. This
second generation of On-Board Diagnostic regulations is called "OBD II" (OBD2).
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. 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 nd and x the problem. Below follow three pieces of such valuable Information:
OBD II Diagnostic Trouble Codes are codes that are stored by the onboard 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 ve-digit alphanumeric code. The rst
character, a letter, identies 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:
On-Board Diagnostics (OBD) II
Diagnostic Trouble Codes (DTCs)
1. Whether the Malfunction Indicator Light (MIL) is commanded 'on' or 'O';
2. Which, if any, Diagnostic Trouble Codes (DTCs) are stored;
3. Readiness Monitor status.
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