Code 25, Mat Sensor Temp. High - 1991 2.5L & 4.3L Turbo: Notes

Fig 1: Code 25, MAT Sensor Circuit Diagram (1991 2.5L & 4.3L Turbo)

Courtesy of GENERAL MOTORS CORP.

CODE 25 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
2.5L
Signal	C12	Tan
MAT Ground	D2	Black/Red
4.3L Turbo
Signal	GF16	Tan
MAT Ground	BB5	Black/Red

The ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, the sensor resistance is high and ECM sees a high signal voltage. As air warms, resistance decreases and voltage sensed by the ECM drops. Sensor resistance can be measured at sensor terminals with harness disconnected.

NOTE: Test numbers refer to test numbers on diagnostic chart.

This checks if the code is a hard failure or an intermittent condition. Code 25 will set if a MAT temperature greater than 266°F (130°C) is sensed for more than a precalibrated period.
This simulates condition for code 23. If control module recognizes the open circuit and "Scan" tester displays a temperature of less than -30°C, control module and wiring are okay.