DFI ECM Tests W/Codes: Action:

ECM turns on "ENGINE CONTROL SYSTEM" light. ECM commands BCM to display "SERVICE SOON" message on CCDIC panel. ECM turns off both AIR management solenoids. ECM uses 40°C for a MAT value when coolant temperature is greater than 40°C. ECM substitutes coolant temperature for MAT when coolant temperature is less than 40°C.

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

The signal voltage from ECM to pin "B" increases as sensor temperature decreases, as 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 the ECM assumes that MAT can't be -35°C to -40°C with coolant temperature of -5°C or more.

1. If sensor is open, ECM parameter ED05 should read -35°C or less. If not, sensor signal is not open at this time. See NOTES ON INTERMITTENTS.
2. Check ECM and sensor circuitry from ECM to sensor connector. If ECM parameter ED05 reads 142-151°C with connector pin "A" shorted to pin "B", sensor circuits and ECM are okay.
3. This steps checks for open sensor ground.
4. This steps checks for ECM's ability to recognize short to ground on MAT input.
5. Fault is most likely at ECM connector or ECM. See CHART C1 - ECM REPLACEMENT CHECK, before ECM is replaced.
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.