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LAUNCH
Catalytic
Converter
Monitor
O
Sensor
2
Monitor
O
Sensor
2
Heater Monitor
EGR Monitor
EVAP system
Monitor
Secondary Air
System
Monitor
2.7 DTCs and MIL Status
When the vehicle's on-board computer detects a failure in an emissions-
related component or system, the computer's internal diagnostic program
assigns a diagnostic trouble code (DTC) that points to the system (and
subsystem) where the fault was found. The diagnostic program saves the code
in the computer's memory. It records a "Freeze Frame" of conditions present
when the fault was found, and lights the Malfunction Indicator Lamp (MIL).
Some faults require detection for two trips in a row before the MIL is turned on.
Note: The "Malfunction Indicator Lamp" (MIL) is the accepted term used
to describe the lamp on the dashboard that lights to warn the driver that an
emissions-related fault has been found. Some manufacturers may still call this
lamp a "Check Engine" or "Service Engine Soon" light.
There are two types of DTCs used for emissions-related faults: Type "A" and
Type "B." Type "A" codes are "One-Trip" codes; Type "B" DTCs are usually
Two-Trip DTCs.
When a Type "A" DTC is found on the First Trip, the following events take
place:
• The computer commands the MIL "On" when the failure is first found.
• If the failure causes a severe misfire that may cause damage to the
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Once per trip
1
Once per trip
1
Once per trip
1
Once per trip
1
Once per trip
1
Once per trip
1
BST580D User's Manual
2
1
3 trips
2
1
3 trips
2
1
3 trips
2
1
3 trips
2
1
3 trips
2
1
3 trips
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