Autel MaxiDiag Elite MD701 - Manual (Page 4)

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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: