Code 23 - Mat Sensor Temperature Low: Notes

The MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM supplies and monitors a voltage signal (4-6 volts) to the sensor through circuit No. 472. When temperatures are low, sensor resistance is high and the ECM will see a high voltage signal. As temperatures increase, sensor resistance decreases and the voltage sensed by the ECM drops.

Code 23 will set if the engine has been running for more than 2 minutes and the signal voltage indicates a MAT temperature of less than 292°F (-34°C). A Code 23 will also result if circuits No. 472 or 469 become open.

1. Checks to see if Code 23 is the result of a hard failure or an intermittent condition.
2. Simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the ECM and wiring are not at fault.
3. Checks for continuity of circuits No. 472 and 469. If circuit No. 469 is open there may also be a Code 33 stored.