With Speed Density Update

Flowchart Code 25:

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 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 MAT sensor is part of the Mass Air Flow (MAF) sensor assembly.



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°C (293°F) for 1 second.

^ 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 grounding CKT 472 will result in a Code 25. Refer to "Intermittents" in Diagnosis by Symptom. - Intermittent Malfunctions