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Onboard Diagnostics
DIAGNOSTIC TROUBLE CODES (DTCs)
Craftsman 14063 15
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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 condi-
tions 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:
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
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.
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: