Powertrain Controls - ECM/PCM

Diagnostic Trouble Code (DTC) 25:

Intake Air Temperature Sensor Circuit:

CIRCUIT DESCRIPTION
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the Powertrain Control Module (PCM). The PCM 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 PCM will sense a high signal voltage. If the intake air is warm, the sensor (thermistor) resistance is low; therefore, the PCM will sense a low signal voltage.

DIAGNOSTIC TROUBLE CODE (DTC) 25 WILL SET WHEN
IAT signal is greater than 151°C (304°F) for 12 seconds and vehicle speed is greater than 5 mph or IAT signal is greater than 145°C (293°F), no vehicle speed and all above conditions met for 60 seconds.

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

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

1. Monitoring IAT with a Tech 1 scanner will determine if the fault is a hard failure or an intermittent condition.

DIAGNOSTIC AIDS
The Tech 1 scanner displays temperature of the air entering the engine and should display close to ambient air temperature, when engine is cold, and rise as underhood temperature increases.

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

Refer to Diagnosis By Symptom/Intermittents. - Intermittent Malfunctions