Mercury MerCruiser MCM 454 Mag MPI Service Manual page 762

SERVICE MANUAL NUMBER 23
Code 25 (2 of 3): Intake Air Temperature (IAT) Sensor Circuit (Non-Scan) High
Temperature Indicated
DIAGNOSTIC AIDS:
Check for the following conditions:
After repairs, clear DTC's following "Clearing Trouble Codes" procedure at the front of this
section. Failure to do so may result in DTC's not properly being cleared.
TEST DESCRIPTION:
Number(s) below refer to step number(s) on the diagnostic table.
STEP 2. This step checks if there is a problem with the ECM and wiring or if the problem
is the IAT sensor.
STEP 3. Check the harness terminals thoroughly for loose connections. If the resistance of
the IAT sensor is monitored, the resistance should steadily decrease as the intake air warms
up. The resistance reading should stabilize when the engine reaches normal operating tem-
perature.
IMPORTANT: If replacing the IAT sensor, tighten hand tight plus 2-1/2 turns maxi-
mum.
Index
90-861326--1 MARCH 1999
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Poor connection at ECM. Inspect harness connectors for backed out terminals,
improper mating, broken locks, improperly formed or damaged terminals, and poor ter-
minal to wire connection.
Damaged harness. Inspect the wiring harness for damage. If the harness appears to be
OK, observe the IAT display on the scan tool while moving connectors and wiring har-
nesses related to the IAT sensor. A change in the IAT display will indicate the location of
the fault.
Check harness routing for a potential short to ground in CKT 472.
TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Page 5G-17