Turbo

Code 25 Chart:

Code 25 Wiring Diagram:

CIRCUIT DESCRIPTION:

The 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 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 the engine is not experiencing turbocharger boost and the following conditions are met:

^ Signal voltage indicates a manifold air temperature greater than 135~C (275~F).

^ The above requirement is met for at least 30 seconds.

^ Due to the conditions necessary to set a Code 25 the "SERVICE ENGINE SOON" light will only remain "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 while the fault is present. If voltage is above 4 volts, the ECM and wiring are OK.

2. If the resistance is less than 100 ohms, replace the sensor.


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 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.