Code EO38, Manifold Air Temperature (Mat) Sensor Too Low: Notes

The MAT sensor measures the air temperature in the air cleaner box and relays this information to the ECM. The ECM applies and monitors a 5-volt signal to the sensor. The sensor is a thermistor that changes its internal resistance as its temperature changes.

When the sensor is cold (internal resistance high), the ECM will see a high signal voltage. This the ECM relates to a cold engine. As the sensor warms (internal resistance decreases), the voltage signal will decrease and the ECM will relate the lower voltage to a warm engine.

As the intake air temperature increases, the sensor resistance becomes less, and the voltage drops.

Code EO38 will set if a signal voltage indicates manifold air temperature less than -30°F (-35°C) for 5 seconds, and the engine running time has been at least 1 minute. Possible causes of a Code EO38 are: Open MAT sensor, faulty wiring or terminals, or a faulty ECM.

Manifold air temperature is one of the inputs used to control: Fuel delivery, spark timing (EST), knock control (ESC), and idle speed (IAC).

1. If the "MAT" reading is -39°C or warmer, then it is a faulty ECM, MAT sensor, or short to voltage in circuit 472.
2. If the voltage at terminal "B" is in the normal 4-6 volt range, then the wiring and ECM are okay. The fault must be a faulty sensor connection or sensor. If over 6 volts, it is a faulty ECM or short to voltage in circuit 472. If short to voltage exits, check for open sensor as a high voltage will destroy the sensor.
3. If the voltage at terminal "B" is in the normal 4-6 volt range, then the wiring and ECM are okay. The fault must be a faulty sensor connection or sensor. If over 6 volts, it is a faulty ECM or short to voltage in circuit 472. If short to voltage exits, check for open sensor as a high voltage will destroy the sensor.
4. If inducing the opposite failure did not produce a reading of 139°C or warmer, then the problem is either in the wiring or ECM. If the voltage at terminal "B" (circuit 472) is in the normal 4-6 volt range, then the problem is either an open in circuit 452, a faulty terminal connection, or a faulty ECM. If voltage at this point is under 4 volts, then it is an open 472 circuit, faulty connection, or faulty ECM.