LG Freezer Error Code 4

Error code 4 on an LG freezer indicates a fault in the freezer defrost sensor circuit, identified in LG documentation as the F-DEF-SENSOR. This sensor is a thermistor responsible for monitoring the temperature of the evaporator or defrost heater assembly during the defrost cycle. The control board uses its resistance readings to determine when the defrost cycle has reached the correct temperature and should terminate. When the main PCB detects that the sensor signal is outside acceptable parameters, either because the circuit is open (broken) or shorted (internally failed or wiring shorted together), it triggers error code 4 and typically halts or limits normal operation.

The primary diagnostic entry point is checking the voltage at the main PCB connector CN30, specifically between pins 3 and 8. Under normal operating conditions, this voltage should fall within the range of 1.0V to 4.5V, varying with temperature as the thermistor changes resistance. A reading at or near 0V typically indicates a short circuit in the sensor or wiring, while a reading at or near the supply rail (above 4.5V) typically indicates an open circuit, meaning the circuit path is broken somewhere between the PCB and the sensor.

Common failure points include the defrost sensor itself degrading internally, wiring harness damage from ice buildup or repeated freeze-thaw cycles, and corrosion at the connector pins. Because this sensor directly governs defrost cycle control, a faulty reading can cause frost accumulation on the evaporator coil over time, which may eventually reduce airflow and cooling performance throughout the freezer compartment.