Code 23: Mat Sensor Temp Low: Notes

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.

1. This checks if Code 23 is because of a hard failure or an intermittent condition. Code 23 will set if engine has been running for a precalibrated period of time and signal voltage indicates a MAT temperature less than -22°F (-30°C).
2. This simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the ECM and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits. If ground circuit is open there may also be a Code 33 or Code 21 stored.