Powertrain Controls - ECM/PCM

Code 25 Chart:

Wiring Diagram For MAT/IAT Sensor:

CIRCUIT DESCRIPTION:

The Manifold Air Temperature (MAT) 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 MAT 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 2 minutes.
c. VSS is greater than 5 mph.


DIAGNOSTIC AIDS:

1. If the engine has been allowed to cool to ambient temperature, coolant and MAT temperatures may be checked with a "Scan" tool. The readings should be close to each other. After the engine starts, the MAT will increase due to Thermic operation and underhood temperatures. If a higher MAT temperature is noted (higher than 80°C), check for proper Thermic operation. See EMISSIONS.
2. A "Scan" tool indicates the temperature of the air surrounding the air cleaner because the MAT sensor is mounted to the air cleaner housing.
3. Carefully check harness routing for possible short to ground in circuit 472. If Code 23 is intermittent, see DIAGNOSIS BY SYMPTOM.