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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
follows 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 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 set. Here
below is an example to illustrate the structure of the digits:
