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Malfunction Indicator Lamp (MIL):
When the vehicle on board computer detects a problem in the emission related
systems or components, its diagnostic program will assign a fault code (DTC) and
store it in its memory. It also records a “Freeze Frame” of the conditions present
when the fault was found and set the malfunction indicator lamp (MIL) alight. Some
faults require detection
for two trips in a row before the MIL is turned on.
Three typical examples of MIL are shown below:
Definition of Trip
‘A Trip’ is define as a Key-ON, Key-OFF event in which the powertrain control
module (PCM) detects the following:
• Engine coolant temperature should exceed 70
o
C
• Engine coolant temperature should change more than 20
o
C after starting the
engine.
• Engine speed should go over 400 RPM.
When the powertain control module (PCM) detects a fault during the 1
st
trip, the DTC
and the corresponding ‘Freeze Frame’ data are stored in the PCM’s memory. The
MIL will not light up until the fault is again detected during the 2
nd
trip. Certain DTCs
are
capable of turning the MIL on or blinking during the first trip.
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