Powertrain Controls - ECM/PCM

Diagnostic Trouble Code 25:

Intake Air Temperature Sensor Circuit:

CIRCUIT DESCRIPTION
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM applies a voltage (about 5 volts) on CKT 472 to the sensor. When intake air is cold, the sensor (thermistor) resistance is high, therefore, the ECM will sense a high signal voltage. If the intake air is warm, the sensor (thermistor) resistance is low, therefore, the ECM will sense a low signal voltage.

TEST DESCRIPTION
Number below refers to circled number on the diagnostic chart.

1. Diagnostic Trouble Code (DTC) 25 will set if:

^ Time since engine start is longer than 5 minutes.
^ Vehicle speed is greater than 44 mph.
^ IAT sensor signal voltage indicates a temperature above 137°C (297°F) for 5 seconds.

DIAGNOSTIC AIDS
IAT on a scan tool indicates the temperature of the air in the air cleaner. If the engine has been allowed to sit overnight, the IAT and engine coolant temperature values should be within a few degrees of each other. After the engine is started, the IAT will increase due to thermostatic air cleaner operation and underhood temperatures. However, the IAT will rarely exceed 80°C (176°F). If a higher IAT than 80°C (176°F) is noted, check for proper thermostatic air cleaner operation. Refer to Fuel Delivery and Air Induction/Air Cleaner Housing. Air Cleaner Housing

Check harness routing for possible short to ground in CKT 472.

Refer to Intermittents in Diagnosis By Symptom. - Intermittent Malfunctions