Powertrain Controls - ECM/PCM

Code 25 Chart:

Intake Air Temperature Wiring Diagram:

CIRCUIT DESCRIPTION:

The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the Electronic Control Module (ECM). The ECM applies a voltage (4-6) 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.

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 150~C (302~F) for 2 seconds.
- Time since engine start is 2 minutes or longer.
- A vehicle speed is present, greater than 5 mph.

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 rise as underhood temperature increases. Check harness routing for possible short to ground in CKT 472. - Intermittent Malfunctions