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

WARNING: This page is about a different car, the 1986 Oldsmobile Toronado and 1986 Buick Riviera. However, it is still accessible from the selected car via links, so may be relevant.

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).

If the "MAT" reading is -39°C or warmer, then it is a faulty ECM, MAT sensor, or short to voltage in circuit 472.
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.
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.
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.