Powertrain Controls - ECM/PCM

Code 25 Chart:

Intake / Manifold Air Temperature Sensor Circuit:

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 (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.


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

1. Code 25 will set if:

- IAT signal voltage is less than or equal to 0.04 volts.
- Signal voltage indicates a intake air temperature greater than 145~C (293~F) for 0.2 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 faulty connection, or an open in CKT 472 will result in a Code 25.