Pcm Code E38, Open Mat Sensor CKT: Description
WARNING: This page is about a different car, the 1992 Cadillac Fleetwood and 1992 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.
MAT sensor is a thermistor (temperature sensitive resistor) that varies resistance with changes in temperature. High temperature will result in low signal voltage. PCM Code E38 sets when PCM sees low voltage signal when coolant temperature is 23°F (-5°C) or greater.