Without Speed Density Update

Chart - Code 25 Manifold Air Temperature (Signal Voltage Low):

Wiring Diagram for Mass Air Flow Sensor Circuit:

CIRCUIT DESCRIPTION
The Manifold Air Temperature (MAT) Sensor uses a thermistor to control the signal voltage to the Electronic Control Module (ECM). The ECM applies a voltage (about 5 volts) on Circuit (CKT) 472 to the sensor. When manifold 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. The Manifold Air Temperature (MAT) Sensor is part of the Mass Air Flow (MAF) Sensor assembly so the ECM can accurately compensate air flow readings based on temperature.

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

1. Code 25 will set if:
^ Signal voltage indicates a manifold air temperature greater than 145 degrees C (293 degrees F) for 3 seconds.
^ Time since engine start is 4 minutes or longer.
^ A vehicle speed is present.

DIAGNOSTIC AIDS
A "Scan" tool reads temperature of the air entering the engine and should read close to ambient air temperature, when engine is cold, and rises as underhood temperature increases. A short to ground in CKT 472 will result in a Code 25. Refer to Diagnosis by Symptom.