Whirlpool Microwave Error Code Err4

Err4 on a Whirlpool microwave indicates that the ambient temperature sensor is reading a value outside the acceptable operating range. This code appears specifically during the diagnostic test mode when the Defrost key is pressed, meaning it is a test-triggered fault rather than one that surfaces during normal cooking operation. The ambient temperature sensor monitors the internal environment of the microwave cavity or control compartment to ensure the unit operates within safe thermal limits.

The sensor itself is typically a thermistor, a resistive component whose resistance changes predictably with temperature. When the control board reads a resistance value that falls too high or too low outside the expected window, it interprets this as either an open circuit, a short circuit, or an actual extreme temperature condition and triggers Err4. A faulty thermistor, a broken or corroded wire harness connection, or a failed control board can all produce this result.

Because the fault is surfaced during diagnostic mode rather than during active use, it is important to distinguish between a sensor that is genuinely out of range due to ambient conditions and one that is electrically faulty. Extremely cold installation environments, such as an unheated garage in winter, can push a functioning sensor outside its rated range. However, in most residential settings, the cause is a defective thermistor or a wiring issue between the sensor and the control board.