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 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:
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:
2
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 States.
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