Equus 3110 - CanOBD2 Diagnostic Tool - Manual (Page 24)

22 CAN OBD2
E
Onboard Diagnostics
DIAGNOSTIC TROUBLE CODES (DTCs)
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 emis-
sions system when the MIL was ordered “On” is saved in the com-
puter’s memory for later retrieval. This information shows fuel sys-
tem status (closed loop or open loop), engine load, coolant tem-
perature, fuel trim value, MAP vacuum, engine RPM and DTC pri-
ority.
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 depend-
ing 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 mem-
ory.
■ 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 misfire 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 condi-
tions” 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.