Powertrain Controls - ECM/PCM

Intake Air Temperature Sensor Circuit Code 25:

Intake Air Temperature (IAT) Sensor:

Circuit Description:
The Intake Air Temperature (IAT) sensor is a thermistor that controls the signal voltage to the ECM. The ECM 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 ECM will see a high signal voltage. As the air warms, the sensor resistance is low, therefore, the ECM will see a low voltage.

Test Description:
Numbers below refer to circled numbers on the diagnostic chart.

1. Code 25 will set if:

^ Signal voltage indicates a Intake Air Temperature (IAT) above 135° C (275° F) for 2 seconds.
^ Time since engine start is 1 minute or longer.
^ A vehicle speed is present.

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.

The "Temperature to Resistance Value" scale 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.