Code 23, Mat Sensor Temp. Low - 1991 2.5L & 4.3L Turbo: Notes

Fig 1: Code 23, MAT Sensor Temp. Low Circuit Diagram (1991 2.5L & 4.3L Turbo)

Courtesy of GENERAL MOTORS CORP.

CODE 23 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 supplies and monitors a voltage signal (4.6 volts) to the sensor. When temperatures are low, sensor resistance is high and the ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by the ECM drops.

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

This checks is Code 23 is a hard failure or an intermittent condition. Code 23 will set if engine is running for one minute, MAT sensor temperature is less than -22°F (-30°C) for 12 seconds and speed sensor signal is not present.
This simulates conditions for a Code 25. If the Scan tester displays a high temperature, the ECM and wiring are not at fault.
This checks for continuity of sensor signal and ground circuits.