Code 23 - Mat Sensor Temperature Low: Notes

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 monitored voltage signal. As temperatures increase, sensor resistance decreases and the voltage sensed by the ECM drops.

1. This step checks to see if Code 23 is the result of a hard failure or an intermittent condition. Code 23 will set if the engine has been running for more than 58 seconds and the signal voltage indicates a MAT temperature less than -22°F (-30°C).
2. Simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the ECM and wiring are not at fault.
3. This step checks for continuity of circuits No. 472 and 469. If circuit No. 469 is open there may also be a Code 33 stored.