Samsung Air Conditioner Error Code 121

Error code 121 on a Samsung air conditioner indicates an open or short circuit fault in the indoor unit's room temperature sensor circuit. The control board cannot get a valid temperature reading from the thermistor used to monitor the ambient air inside the room. The fault is typically caused by a loose connector, a broken wire in the sensor harness, or a failed thermistor sensor. The unit will not operate normally until this fault is resolved.

It is not recommended to continue running the air conditioner while error code 121 is active. Without a functioning room temperature sensor, the control board cannot accurately regulate the unit's operation, which could result in inefficient performance or unintended behavior. The unit will typically shut down on its own to prevent issues, and forcing operation is not advised.

A power reset may temporarily clear the error code display, but if the underlying wiring or sensor fault is still present, the code will return once the unit restarts and the control board attempts to read the sensor again. A reset is useful for confirming whether the fault is intermittent, such as a connector that was briefly loose, but it is not a repair. The sensor circuit should be inspected and tested to find and fix the actual cause.

Some causes of error code 121 are accessible to a knowledgeable DIYer. Checking for a disconnected or loose sensor connector and inspecting the wire harness for visible damage can be done without special tools. If the connector is properly seated and the wiring looks intact, testing the thermistor with a multimeter requires basic electrical knowledge. Replacing the sensor or control board, if needed, may be better handled by a certified technician depending on your comfort level.

If the fix is simply reseating a loose connector, there may be no parts cost at all, though a service call fee will still apply if you use a technician. A replacement room temperature thermistor sensor is generally an inexpensive part, typically in the range of $10 to $40 depending on the model. If the control board itself is at fault, replacement costs are significantly higher, often in the range of $150 to $350 or more including parts and labor.