GE Microwave Error Code Lower Thermal Sensor OPEN

This error means the Smart Board has detected an open circuit in the lower thermistor, the sensor responsible for monitoring temperature in the lower section of the microwave. The board expects to read approximately 256K ohms of resistance from this sensor at room temperature via the red and white wires at connector CN09. An open circuit reading indicates the sensor has failed, a wire has broken, or the connector has come loose. The microwave will not operate safely until the fault is corrected.

No, you should not operate the microwave with this error active. The lower thermal sensor is part of the overheat protection system, and without a functioning sensor the microwave cannot properly monitor or limit internal temperatures. Continued use could result in overheating, component damage, or a fire hazard. The appliance should remain unplugged until the fault is diagnosed and repaired.

A power reset, such as unplugging the unit for a few minutes, may temporarily clear the error display, but the fault will return as soon as the Smart Board re-checks the thermistor circuit and again detects an open condition. Because this error reflects an actual hardware fault rather than a software glitch, the underlying problem with the sensor, wiring, or connector must be physically corrected before the error will clear permanently.

A knowledgeable DIYer with a multimeter can perform the initial diagnostic by checking resistance across the thermistor leads at CN09 with the unit unplugged. If the connector is simply loose, reseating it may resolve the issue without any parts. However, if the thermistor or wiring needs replacement, some disassembly of the microwave cabinet is required, and high-voltage capacitors inside the unit can retain a dangerous charge even after the power is disconnected, so proper discharge procedures are essential before working inside.

If only the thermistor needs replacement, parts typically cost in the range of $15 to $50 depending on the specific model. If professional labor is involved, a repair visit can add $75 to $150 or more in service fees. Wiring harness repairs fall in a similar range. If the Smart Board itself is the root cause, board replacement costs can range from $100 to $250 or higher for parts alone, which may make repair less economical on older units.