Action Taken

WARNING: This page is about a different car, the 1990 Cadillac Fleetwood and 1990 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.

ECM turns on SERVICE SOON light.
ECM uses 104°F (40°C) for a MAT value when coolant temperature is greater than 104°F (40°C).
ECM substitutes coolant temperature for MAT when coolant temperature is less than 104°F (40°C).

MAT sensor is a thermistor (temperature sensitive resistor) that varies resistance with changes in temperature. MAT sensor is a 2-wire sensor with a monitored reference voltage coming from ECM to sensor terminal "B", circuit No. 472, and a sensor reference ground on terminal "A", circuit No. 469. As temperature of sensor increases, sensor resistance decreases.

The monitored signal voltage from ECM to terminal "B" decreases as sensor temperature increases because more current flows from sensor terminal "B" through sensor element to ground. Less of signal voltage is dropped across sensor element.

Code E38, open MAT sensor, sets because ECM assumes that MAT cannot be -26° to -40°F (-32° to -40°C) with a coolant temperature of 23°F (-5°C) or greater.

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

Test Number 1) 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 NOTE ON INTERMITTENTS.

Test Number 2) Checks ECM and sensor circuitry from ECM to sensor connector. If ECM data ED05 reads 293°-304°F (148°-151°C) with connector terminal "B" shorted to terminal "A", the sensor circuits and ECM are okay.

Test Number 3) Checks for open sensor ground.

Test Number 4) Checks for ECM ability to recognize short to ground on MAT input.

Test Number 5) Fault is most likely at ECM connector or ECM. Before ECM is replaced, perform CHART C-1: ECM REPLACEMENT CHECK .

Test Number 6) 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.