Code 23A, Manifold Air Temperature (Mat) Sensor Circuit Low Temperature Indicated: Notes

The MAT sensor uses a thermistor to control signal voltage to ECM. The ECM applies 4-6 volts on Purple/Black wire to sensor. When manifold air is cold, the sensor (thermistor) resistance is high and the ECM sees a high voltage. As the air warms, the sensor resistance becomes less and the voltage drops.

1. This checks if Code 23A resulted from a hard failure or an intermittent condition. Code 23 sets if engine is running longer than 2 minutes and signal voltage indicates a MAT temperature below -86°F (-30°C).
2. This simulates conditions for a Code 23B. If Scan tester displays a high temperature, the ECM and wiring are okay.
3. This step check continuity of Purple/Black wire and Black/White wire. If Black/White wire is open, a Code 21 may be present.