LG Air Conditioner Error Code 121

Error code 121 indicates that the discharge temperature sensor for constant speed compressor 1 on the sub1 outdoor unit has been detected as open or shorted. This sensor sits on the compressor discharge pipe and monitors refrigerant exit temperature. The system shuts down when the sensor signal is out of range to protect the compressor from operating in an unmonitored state. The fault points to either a wiring problem or a failed sensor.

No, you should not attempt to bypass or ignore this fault. The discharge temperature sensor protects the compressor from overheating, and operating without a functioning sensor means the system cannot detect a dangerously high discharge temperature condition. Continued operation could result in compressor failure, which is a costly repair.

A power reset may temporarily clear the fault code, but if the underlying issue with the sensor or wiring has not been resolved, the error will return as soon as the system performs its startup diagnostics. Resetting without repairing the root cause can also delay diagnosis and risk compressor damage. The fault should be properly diagnosed and repaired before returning the unit to normal operation.

A knowledgeable DIYer can perform initial checks such as inspecting the sensor lead wire for visible damage and verifying the connector is firmly seated. However, accessing the outdoor unit control board and testing sensor resistance values requires comfort with electrical diagnostics and multimeter use. Any work involving refrigerant or internal compressor components should be left to a licensed HVAC technician.

If the fix is simply reseating a loose connector, there may be no parts cost and only a service call fee, typically in the range of $75 to $150. Replacing the discharge temperature sensor itself is generally a low-cost repair, with parts ranging from $20 to $80 and labor adding to the total. If the outdoor control board has failed, repair costs can rise significantly, often between $200 and $500 or more depending on the unit model.