Code 23, Manifold Air Temperature (Mat) Sensor (Temperature Too Low): Notes

The MAT sensor uses a thermistor type resistor to control signal voltage to the ECM. A 4-6 volt reference signal is supplied to the sensor on circuit on circuit No. 472. When manifold air temperature is cold, sensor resistance is high and the ECM will see a high signal voltage. As the manifold air temperature increases, sensor resistance becomes less and voltage drops.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) This step checks if Code 23 is a result of a hard failure or an intermittent condition.

A Code 23 will set if:

Signal voltage indicates a MAT temperature of less than -31°F (-35°C) with engine running longer then 2 minutes.
With engine off, MAT reading is less than -31°F (-35°C) while the coolant temperature is greater than 15°F (-26°C).

Test Number 2) This test simulates conditions for a Code 25 (high MAT temperature). If the Scan tester displays a high temperature, the ECM and wiring are okay.

Test Number 3) This step checks continuity of circuits No. 472 and 452. If circuit No. 452 is open, there may also be a Code 33 set.