Action Taken
- ECM turns on "ENGINE CONTROL SYSTEM FAULT" light.
- ECM commands BCM to display "SERVICE SOON" message on DIC.
- ECM disables AIR system.
- 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).
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 the ECM to sensor pin "B" circuit No. 472 and a sensor reference ground on pin "A" circuit No. 476.
As temperature of sensor increases, sensor resistance is lower. The signal voltage from ECM to pin "B" decreases as sensor temperature increases, because current flows from pin "B" through the sensor element to pin "A" ground sensor. Voltage is dropped across the sensor element.
High temperature means low signal voltage on circuit No. 472, and low temperature means high signal voltage on circuit No. 472. Code EO38 sets when ECM sees a low MAT reading when coolant temperature is 23°F (-5°C).
- If sensor is open, ECM parameter ED05 should read -31°F (-35°C) or less. If not, sensor signal is not open at this time.
- Checks ECM and sensor circuitry from ECM to sensor connector. If ECM parameter ED05 reads 293-304°F (148-151°C) with connector pin "A" shorted to pin "B", the sensor circuits and ECM are okay.
- Checks for open sensor ground.
- Checks for ECM ability to recognize short to ground on MAT input.
- Fault is at ECM connector or ECM. Before ECM is replaced, perform PFI CHART C-1: ECM REPLACEMENT CHECK .
- 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.