LG HVAC Error Code E121

E121 indicates an indoor temperature sensor error caused by a short or open circuit in the sensor circuit. The system uses this sensor to monitor indoor ambient temperature and control operation. When the sensor signal is out of range or absent, the unit logs E121 and stops normal operation to prevent running without accurate temperature data.

The unit will typically shut down or refuse to operate normally when E121 is active, precisely because it cannot regulate temperature without a working sensor. Attempting to force operation without resolving the fault is not recommended, as the system lacks the data needed to control heating or cooling cycles safely. Address the underlying sensor or wiring issue before returning the unit to service.

If the fix is simply reseating the CN41 connector, there may be no parts cost at all, though a service call will typically run between $75 and $150 for labor. If the indoor temperature sensor thermistor needs replacement, parts generally range from $15 to $60 depending on the specific model. A PCB replacement, if required, is significantly more expensive and can range from $150 to $400 or more for parts alone.

Start by checking whether the sensor connector at CN41 on the indoor PCB has come loose, and firmly reseat it if so. If the connector is secure, use a multimeter to test the resistance of the temperature sensor to determine if it has shorted or failed open. A damaged wiring harness or corroded connector pins can also cause this error and should be inspected. If the sensor or wiring is faulty, replacing the defective component is required.

A power reset may temporarily clear the error display, but the fault will return immediately if the underlying sensor or wiring problem has not been corrected. A reset is only useful as a final step to confirm the repair after the sensor, connector, or wiring has been inspected and any issues resolved. Repeated resets without repair will not fix the problem.