Diagnostic Aids:

The MAT sensor is a thermistor or variable resistor that varies in resistance with temperature. The MAT sensor is a 2-wire sensor with a reference or signal voltage coming from ECM to sensor pin B circuit No. 472, and a sensor reference ground on pin A circuit No. 476. As temperature of sensor decreases, sensor resistance increases.

The signal voltage from ECM to pin B increases as sensor temperature decreases because less current flows from sensor pin B through sensor element to ground. Less of signal voltage is dropped across sensor element. Low temperature means high signal voltage on circuit No. 472, and high temperature means low signal voltage on circuit No. 472.

Code "EO38", open MAT sensor, sets because ECM assumes that MAT can not be -31 to -40°F (-35 to -40°C) with a coolant temp of 23°F (-5°C) or greater.

NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

If sensor is open, ECM parameter P.0.5. should read -31°F (-35°C) or less. If not, sensor signal is not open at this time. See NOTES ON INTERMITTENTS.
Check ECM and sensor circuitry from ECM to sensor connector. If ECM parameter P.0.5. reads 293-304°F (148-151°C) with connector pin A shorted to pin B, sensor circuits and ECM are okay.
Checks for open sensor ground.
Checks for ECM ability to recognize a short to ground on MAT input.
Fault is at ECM connector or ECM. Before ECM is replaced, perform PERFORMANCE CHART C1 - ECM REPLACEMENT CHECK at the end of this article.
MAT sensor can be damaged by backfire in intake, or by excessive intake heat due to valve train faults. If vehicle has had multiple MAT sensor replacements, check for signs of backfire or high intake manifold air temperature due to improper valve train operation.