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. This check determines if the Code 25 is a result of a hard failure or an intermittent condition. A Code 25 will set if the following exist:
a. The IAT temperature is greater than 150°C (302°F) and is detected for time greater than two seconds.
b. The engine has been running longer than 5 minutes.
c. VSS is greater than 35 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), check for proper THERMAC operation. See EMISSIONS CONTROL SYSTEMS.

^ A "Scan" tool indicates the temperature of the air surrounding the air cleaner because the MAT sensor is mounted to the air cleaner housing.

^ Carefully check harness routing for possible short to ground in circuit 472. If Code 23 is intermittent, - Intermittent Malfunctions