LG Air Conditioner Error Code E121

E121 indicates a fault with the indoor room temperature sensor inside the indoor unit, detected as either a short circuit or an open circuit condition. The sensor is a thermistor that provides real-time room temperature data to the control board. When the board reads a signal outside the acceptable resistance range, it locks out normal operation and displays E121. The fault can be in the sensor itself, its connector, or the wiring harness connecting it to the PCB.

You should not continue running the unit while E121 is active. Without a functioning room temperature sensor, the control board cannot accurately regulate the system, which could lead to improper operation or component stress. LG units typically disable normal operation when this fault is detected for this reason. Address the sensor fault before resuming use.

A power reset may temporarily clear the error display, but E121 will return immediately if the underlying sensor fault has not been corrected. The control board continuously monitors the sensor signal during operation and will re-trigger the fault code as soon as it detects an out-of-range resistance reading. A permanent fix requires reseating the connector, repairing or replacing damaged wiring, or replacing the faulty sensor.

A knowledgeable DIYer can start by checking whether the sensor connector is firmly seated on the PCB and visually inspecting the wiring for obvious damage. If you have a multimeter, measuring resistance across the sensor terminals can confirm whether the thermistor has failed. Replacing the sensor is a relatively straightforward repair if you are comfortable working inside the indoor unit, but if the PCB itself is suspected, professional diagnosis is recommended.

If the fix is simply reseating a loose connector, there is no parts cost and minimal labor time involved. Replacing the indoor room temperature sensor typically involves parts costs in the range of $10 to $40 depending on the specific unit model. If a service technician is required for diagnosis and repair, total service costs including labor could range from $75 to $200 or more depending on your location and the complexity of accessing the sensor.