Powertrain Controls - ECM/PCM

Code 25 Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION:

The Intake Air Temperature (IAT) sensor is a thermistor. The ECM applies a voltage (about 5.0 volts) on CKT 472 to the sensor. When air is cold, the sensor (thermistor) resistance is high, therefore, the ECM will measure a high signal voltage. If the air is warm, the sensor resistance is low, and the ECM will measure a low voltage.

NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to ECM CONNECTOR IDENTIFICATION under ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS for correct color code identification of circuit.

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

Code 25 will set If:

^ Signal voltage indicates an intake air temperature greater than 135°C (275°F).
^ Vehicle speed is greater than 35 mph.
^ Both of the above requirements are met for at least 16 seconds.
^ Due to the conditions necessary to set a Code 25, the "SERVICE ENGINE SOON" light will only stay "ON" while the fault is present.

1. A "Scan" tool may not be used to diagnose this fault due to the ECM transmitting "default" (substitute) values when the fault is present. If voltage is above 4 volts, the ECM and wiring are OK.

2. If the resistance is less than 185 ohms, replace the sensor.