Pcm Code EO38, Open Mat Sensor Circuit: Action

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

1. If sensor is open, PCM parameter ED05 should read -35°C or less. If not, sensor signal is not open at this time. See NOTE ON INTERMITTENTS.
2. Check PCM and sensor circuitry from PCM to sensor connector. If PCM parameter ED05 reads 148-151°C with connector terminal "A" shorted to terminal "B", sensor circuits and PCM are okay.
3. This step checks open sensor ground.
4. This step checks PCM's ability to recognize short to ground on MAT input.
5. Fault is most likely at PCM connector or PCM. See PFI CHART C-1, PCM REPLACEMENT CHECK before PCM 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 signs of backfire or high intake manifold air temperature due to improper valve train operation.