Powertrain Controls - ECM/PCM

DTC 25 Intake Air Temperature (IAT) Sensor Circuit (High Temperature Indicated):

Intake Air Temperature (IAT) Sensor Circuit (Low Temperature Indicated):

CIRCUIT DESCRIPTION
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the Powertrain Control Module (PCM). The PCM supplies a 5V reference and a ground to the sensor. When intake air is cold, the sensor (thermistor) resistance is high, therefore, signal voltage will be high. As the air warms, the sensor resistance decreases, and the voltage drops. As the incoming air gets warmer, the sensor resistance decreases, causing signal voltage to decrease.


DIAGNOSTIC TROUBLE CODE (DTC) 25 WILL SET WHEN
^ IAT signal voltage indicates intake air temperature greater than about 151 °C (304°F).
^ Vehicle speed is greater than 15 mph.
^ Engine run time is greater than 320 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
The Malfunction Indicator Lamp (MIL) will illuminate, the PCM will default to 60°C for intake air temperature (Tech 1 scan tool will not display defaulted value), and the cooling fan will enable.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step checks if DTC 25 was set as an intermittent or a hard condition.

DIAGNOSTIC AIDS
^ The "Temperature vs. Resistance Value" scale in the image 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 can result in poor driveability.
^ If engine is at ambient temperature compare IAT sensor to Engine Coolant Temperature (ECT) sensor. They should be within a couple degrees of each other, or compare against a known good IAT sensor.
^ Refer to "Intermittents," in Diagnosis by Symptom. - Intermittent Malfunctions