Powertrain Controls - ECM/PCM

Code 25 Chart:

Wiring Diagram For MAT/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 reference voltage (approximately 5.0 volts) on circuit 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 becomes less, and the voltage drops.


TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.

1. A Code 25 will set if the following exist:
^ The IAT sensor signal voltage indicates a temperature greater than 135°C (275°F).
^ The engine has been running longer than 5 minutes.
^ VSS is greater than 45 mph.


DIAGNOSTIC AIDS:

^ If the engine has been allowed to cool to ambient temperature, coolant and IAT temperatures may be checked with a "Scan" tool. The readings should be close to each other. After the engine starts, the IAT will increase due to THERMAC operation and underhood temperatures. If a higher IAT temperature is noted, higher than 80°C (176°F), check for proper THERMAC operation. See EMISSION CONTROLS.
^ A "Scan" tool indicates the temperature of the air surrounding the air cleaner because the IAT sensor is mounted to the air cleaner housing.
^ Carefully check harness routing for possible short to ground in circuit 472. If Code 25 is intermittent, - Intermittent Malfunctions