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18
LAUNCH
Professional 909 User Manual
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 rst found.
• If the failure causes a severe misre that may cause damage to the catalytic
converter, the MIL “ashes” once per second. The MIL continues to ash as
long as the condition exists. If the condition that caused the MIL to ash is no
longer present, the MIL will light “steady” On.
• A DTC is saved in the computer’s memory for later retrieval.
• 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:
• 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.
• 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.
• 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:
• 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 misre faults). The DTCs are automatically erased if the
fault that caused them to be set is not detected again during that period.
• Misfire and fuel system faults require three trips with “similar conditions”
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