Samsung Air Conditioner Error Code 121

Samsung air conditioner error code 121 indicates a fault in the indoor unit's room temperature sensor circuit. This thermistor sensor monitors the ambient air temperature inside the conditioned space and continuously feeds that data to the control board. The board uses this input to regulate compressor operation, fan speed, and target temperature cycling. When the control board detects a signal that falls outside the expected resistance range, either because the circuit is open or shorted, it triggers error code 121 and halts normal operation to prevent incorrect temperature management.

The three primary hardware causes are connector dislocation, wire breakage, and a defective sensor itself. A dislodged connector at either the sensor end or the control board end will interrupt the signal entirely, producing an open circuit condition. A broken or pinched wire along the sensor harness produces the same result. A failed thermistor, where the sensor's internal resistance has drifted far out of specification or collapsed entirely, creates either an open or short condition depending on the failure mode.

Diagnostic work should begin at the wiring. Visually inspect the sensor harness for any cuts, pinching, or abrasion, then confirm that the connector is fully seated at both connection points. If the wiring checks out, resistance testing of the thermistor itself with a multimeter will confirm whether the sensor is within the expected range for the ambient temperature at the time of testing. A reading of zero ohms indicates a short, while an infinite resistance reading indicates an open circuit, both of which confirm sensor replacement is needed.