ECM Code EO38: Open Mat Sensor Circuit: Action:

ECM turns on SERVICE ENGINE 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 temperature when coolant temperature is less than 104°F (40°C).

The MAT sensor is a thermistor that varies its resistance with temperature. The MAT sensor is a 2-wire sensor with a monitored 5-volt reference signal 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 decreases, sensor resistance increases.

The monitored signal voltage from ECM to terminal "B" increases as sensor temperature decreases, as less current flows from sensor terminal "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. High temperature means low signal voltage on circuit No. 472.

Code EO38, open MAT sensor, sets because ECM assumes that MAT cannot be -31°F to -40°F (-35°C 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 ED05 should read -35°C or less. If not, sensor signal is not open at this time. See Note on Intermittents.

Test Number 2) Check ECM and sensor circuitry from ECM to sensor connector. If ECM parameter ED05 reads 148-151°C with connector terminal "A" shorted to terminal "B", sensor circuits and ECM are okay.

Test Number 3) This steps checks for open sensor ground.

Test Number 4) This steps checks ECM's ability to recognize short to ground on MAT input.

Test Number 5) Fault is most likely at ECM connector or ECM. See CHART C1, ECM REPLACEMENT CHECK before ECM is replaced.

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.