Page 347 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

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.

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:
1 The computer commands the MIL “On” when the failure is first found.
2 If the failure causes a severe misfire that may cause damage to the catalytic converter, the MIL “flashes” once
per second. The MIL continues to flash as long as the condition exists. If the condition that caused the MIL to
flash is no longer present, the MIL will light “steady” On.
3 A DTC is saved in the computer’s memory for later retrieval.
4 A “Freeze Frame” of the conditions present in the engine or emissions system when the MIL was ordered “On”
is saved in the computer’s memory for later retrieval. This information shows fuel system status (closed loop
or open loop), engine load, coolant temperature, fuel trim value, MAP vacuum, engine RPM and DTC priority.
When a Type “B” DTC is found on the First Trip, the following events take place:
1 The computer sets a Pending DTC, but the MIL is not ordered “On.” “Freeze Frame” data may or may not be
saved at this time depending on manufacturer. The Pending DTC is saved in the computer’s memory for later
retrieval.

2 If the failure is found on the second consecutive trip, the MIL is ordered “On.” “Freeze Frame” data is saved in
the computer’s memory.
3 If the failure is not found on the second Trip, the Pending DTC is erased from the computer’s memory.
The MIL will stay lit for both Type “A” and Type “B” codes until one of the following conditions occurs:

1 If the conditions that caused the MIL to light are no longer present for the next three trips in a row, the computer
automatically turns the MIL “Off” if no other emissions-related faults are present. However, the DTCs remain
in the computer’s memory as a history code for 40 warm-up cycles (80 warm-up cycles for fuel and misfire
faults). The DTCs are automatically erased if the fault that caused them to be set is not detected again during
that period.
2 Misfire and fuel system faults require three trips with “similar conditions” before the MIL is turned “Off.” These
are trips where the engine load, RPM and temperature are similar to the conditions present when the fault was
first found.
After the MIL has been turned off, DTCs and Freeze Frame data stay in the computer’s memory.

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CITS : Automotive - Mechanic Diesel - Lesson 104 - 107