LG Microwave Error Code E-21

The E-21 error code on an LG microwave indicates a Temperature Sensor Open Error. This fault is triggered when the control board reads a sensor value exceeding 250, which falls outside the acceptable operating range. When this threshold is crossed, the microwave immediately exits its current operation and enters cancel mode, preventing the unit from continuing to run under a potentially faulty thermal monitoring condition. The temperature sensor in question is typically a thermistor responsible for monitoring internal cavity or component temperatures during cooking cycles.

The most likely cause of this error is an open circuit condition in the temperature sensor circuit. An open circuit means the electrical path through the thermistor has been broken, causing the resistance to read at an extreme value that the PCB interprets as a sensor value above 250. This can result from a failed thermistor, a loose or disconnected connector at the sensor housing, or damaged wiring between the sensor and the main PCB. Corrosion at connector terminals is another common contributor, particularly in units that have been exposed to moisture or steam over time.

Diagnosing this fault begins with a physical inspection of the sensor component itself and the connector linking the sensor housing to the PCB. Technicians should check for visible damage, connector seating, and continuity through the thermistor. If the sensor reads an open circuit on a multimeter, the thermistor has failed and requires replacement. If continuity is present at the sensor but the error persists, the wiring harness and PCB connector pins should be closely inspected for intermittent contact or corrosion.