Indoor unit displays E121 and stops; fan may still respond.

Safety warning: Disconnect power before removing the indoor unit front cover.

• Step 1 – Access the sensor: Remove the front panel and air filters. Locate the room temperature sensor (small probe usually near the air intake grille, connected by thin wires to the PCB).
• Step 2 – Check connections: Ensure the thermistor plug is firmly seated on the indoor PCB. Reseat the connector.
• Step 3 – Inspect sensor: Look for damage to the sensor tip or cut/chewed wires.
• Step 4 – Replace if faulty: If the sensor is physically damaged or E121 persists, replace the room temperature sensor with a Samsung-compatible thermistor for your model.
• Step 5 – Test: Restore power and run the unit. If the code clears, the repair is successful. If not, the indoor PCB may also be faulty.