Bosch Refrigerator Error Code F4

The F4 error code on a Bosch refrigerator indicates a short circuit condition in the evaporator temperature sensor located in the refrigerator compartment. This sensor is a negative temperature coefficient (NTC) thermistor that monitors the evaporator coil temperature and sends resistance-based readings to the electronics control module. A short circuit means the sensor's resistance has dropped to near zero, causing the control board to receive an out-of-range signal that it cannot interpret as a valid temperature reading.

When the evaporator sensor short-circuits, the refrigerator's control system loses its ability to accurately regulate cooling cycles in the fresh food compartment. The electronics module relies on this sensor input to determine when to run the compressor and fans, so a faulty reading can result in continuous cooling, insufficient cooling, or a complete shutdown of the cooling cycle depending on how the control logic handles the fault condition.

The first diagnostic step is to inspect the sensor itself and trace the supply cable running from the sensor back to the electronics module. Damage along the wiring harness, such as pinched insulation, moisture intrusion at a connector, or a wire shorted against a metal component, can produce the same fault code as a failed sensor. Resistance testing of the sensor at room temperature will quickly reveal whether the thermistor itself has failed internally or whether the fault lies in the wiring between the sensor and the control board.