Code 23 - Mat Sensor (Low Temperature Indicated): Notes

The Manifold Air Temperature (MAT) sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies and monitors voltage on circuit No. 472 to the sensor. When the air is cold, sensor resistance is high and the ECM will see a high monitored voltage. If the air is warm, the sensor resistance is low and ECM will see a low monitored voltage.

1. Code 23 will set if signal voltage indicates manifold air temperature is low, time since engine start is more than 4 minutes and no vehicle speed signal is present (vehicle not moving). Due to the conditions necessary to set a Code 23, the "SERVICE ENGINE SOON" light will only stay on when all 3 conditions are met.
2. A Code 23 will set due to an open sensor, wire or connection. This test will determine if the wiring and ECM are okay. The MAT sensor is difficult to reach on some models so this test can be performed at the MAT sensor harness connector.
3. This will determine if an open is present in the monitored signal circuit No. 472 or the 5-volt return circuit No. 452.