DTC 25

Code 25 Chart:

IAT Or MAT Sensor Wiring Diagram:

CIRCUIT DESCRIPTION:

The IAT or MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage (about 5 volts) on CKT 472 to the sensor. When the air is cold the sensor (thermistor) resistant is high therefore the ECM will see a high signal voltage. If the air is warm the sensor resistance is low and the ECM will see a low voltage.

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


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 135~C (293~F) for 0.2 seconds.
^ A vehicle speed over 1 mph is present.
^ Due to the conditions necessary to set a Code 25 the "SERVICE ENGINE SOON" light will remain "ON" while the signal is low and 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 rise as underhood temperature increase. A short to ground in CKT 472 will result in a Code 25. The "Temperature to Resistance Values" scale may be used to test the IAT or MAT sensor at various temperature levels to evaluate the possibility of a "shifted" (mis-scaled) sensor. a "slewed" sensor could result in poor driveability complaints. Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/INTERMITTENTS.