LG Air Conditioner Error Code 129

Error code 129 on an LG multi-unit air conditioner indicates an open circuit or short circuit in the low pressure sensor of the Sub2 outdoor unit in a three-unit system. The low pressure sensor monitors refrigerant pressure on the suction side of the system. When the sensor circuit reads outside its expected resistance range, the system logs this fault and may restrict compressor operation to prevent damage.

It is not recommended to continue running the system with error code 129 active. The low pressure sensor is a key protection device for the compressor, and without accurate pressure readings, the system cannot protect itself from low refrigerant or other abnormal operating conditions. Running the system in this state risks compressor damage, which is a costly repair.

A power reset may temporarily clear the error code display, but the fault will return quickly if the underlying sensor or wiring problem has not been resolved. Because the control board continuously monitors the sensor circuit during operation, any remaining open or short circuit condition will retrigger the error. The root cause must be identified and corrected for the code to clear permanently.

Some basic checks can be performed by a knowledgeable DIYer, including inspecting the sensor connector for looseness or corrosion and verifying the wiring harness is not visibly damaged. However, accurately measuring sensor resistance and diagnosing control board faults requires a multimeter and familiarity with HVAC electrical circuits. Any work involving refrigerant or sealed system components must be performed by a certified technician.

Repair costs depend on the root cause. Cleaning or reseating a corroded connector may cost only a service call fee, typically in the range of $75 to $150. Replacing the low pressure sensor usually adds $50 to $200 in parts depending on the unit model. If the control board needs replacement, total costs including labor can range from $300 to $700 or more.