Powertrain Controls - ECM/PCM

DTC 25 Chart CMFI:

IAT Circuit CMFI:

CIRCUIT DESCRIPTION:
The Intake Air Temperature (IAT) sensor is a thermistor that controls the signal voltage to the PCM. The PCM applies a voltage (4-6 volts) on CKT 472 to the sensor. When the air is cold, the sensor (thermistor) resistance is high, therefore, the PCM will see a high signal voltage. As the air warms, the sensor resistance is low, therefore, the PCM will see a low voltage.

DTC 25 WILL SET WHEN:
^ Signal voltage indicates an Intake Air Temperature (IAT) above 130° C (270° F) for 25 seconds.
^ Time since engine start is 4 minutes or longer.
^ A vehicle speed is present.

ACTION TAKEN (PCM WILL DEFAULT TO):
The Malfunction Indicator Lamp (MIL) will illuminate.

DTC 25 WILL CLEAR WHEN:
Conditions for fault are no longer present and Tech 1 scan tool clear DTCs function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and ECM "B" fuse is removed for 10 seconds.

DTC CHART TEST DESCRIPTION:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Verifies IAT sensor display and checks sensor and wiring.

DIAGNOSTIC AIDS:
A Tech 1 scan tool reads temperature of the air entering the engine and should read close to ambient air temperature when engine is cold, and rises as underhood temperature increases.
Check harness routing for possible short to ground in CKT 472.
Refer to Diagnosis By Symptom/Intermittents - Intermittent Malfunctions
The "Temperature to Resistance Value" scale within the diagnostic chart may be used to test the IAT sensor at various temperature levels to evaluate the possibility of a "skewed" (mis-scaled) sensor. A "skewed" sensor could result in poor driveability complaints.