Powertrain Controls - ECM/PCM

DTC 25 Chart:

Intake Air Temperature Sensor Circuit:

Circuit Description:
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the PCM. The PCM applies a voltage (about 5 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. If the air is warm the sensor resistance is low, therefore the PCM will see a low voltage.

The IAT sensor is located in the rear air intake duct.

DTC 25 Will Set When:
Vehicle speed is greater than 1 mph and intake air temperature indicates 145°C (297°F) for 2 seconds.

Action Taken (PCM will default to):
The PCM will use an IAT default value of 37°C (108°F) and the MIL will become illuminated.

DTC 25 Will Clear When:
A current DTC 25 will clear when the intake air temperature is indicated below the high temperature threshold. A history DTC 25 will clear after 50 consecutive ignition key cycles without a current DTC 25 being stored.

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. This test is a verification that the DTC is not intermittent.

2. This test helps determine between a circuit fault and a faulty sensor.

Diagnostic Aids:
A Tech 1 scan tool reads the temperature of the air entering the engine and should read close to ambient air temperature when engine is cold. IAT values will rises as underhood temperature increases.

A short to ground in CKT 472 will result in a DTC 25.

The "Temperature to Resistance Values" scale at the right may be used to test the IAT sensor at various temperature levels to evaluate the possibility of a "shifted" (mis-scaled) sensor. A "skewed" sensor could result in poor driveability complaints.

Refer to "Intermittents" in "Diagnosis By Symptom." - Intermittent Malfunctions