Action Taken

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

The MAT sensor is a thermistor (temperature sensitive resistor) that varies resistance with changes in temperature. The MAP sensor is a 2-wire sensor with a monitored reference voltage coming from the 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 is lower. The signal voltage from ECM to terminal "B" decreases as sensor temperature increases because current flows from terminal "B" through the sensor element to terminal "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 E37, shorted MAT sensor, sets because ECM assumes that MAT cannot be 291°F (144°C) or greater with a coolant of less than 212°F (100°C).

Test Number 1)  With a shorted sensor, ECM parameter P.0.5 should read 298°F (148°C) or greater. If not, sensor is not shorted. See NOTE ON INTERMITTENTS.

Test Number 2)  Checks for sensor shorted or circuit No. 472 shorted. If ECM parameter P.0.5 stays at 288°-304°F (142°-151°C) with sensor unplugged, short is in circuit No. 472.

Test Number 3)  MAT sensors can be damaged by a backfire in intake. If vehicle has had more than one MAT sensor replaced, check for signs of backfire or high intake manifold temperatures due to improper valve train operation.

Test Number 4)  Fault is most likely at ECM connector or ECM. Before ECM is replaced, perform CHART C-1: ECM REPLACEMENT CHECK  .