LG Air Conditioner Error Code 128

Error code 128 indicates that the Sub2 outdoor unit high pressure sensor has an open or short circuit condition in a three-unit system configuration. The high pressure sensor monitors refrigerant pressure on the discharge side of the outdoor unit, and this fault means the system cannot get a valid reading from that sensor. The outdoor unit may shut down or limit operation to protect the system until the fault is resolved.

Running the system with this fault active is not recommended. The high pressure sensor is a safety component that helps prevent damage from excessive refrigerant pressure. Without a reliable reading from this sensor, the outdoor unit cannot properly protect itself from high-pressure conditions. Continued operation could risk compressor or component damage.

A power reset may temporarily clear the fault code, but if the underlying sensor open or short circuit condition still exists, the error will return as soon as the system attempts to read the sensor again. A reset without repair does not address the root cause. Persistent reappearance of code 128 after a reset confirms that inspection and repair of the sensor circuit are necessary.

Some steps are DIY-accessible, such as inspecting and reseating the sensor wiring harness connector and checking for visible wire damage. Measuring sensor resistance with a multimeter is also straightforward if you have basic electrical knowledge. However, replacing the high pressure sensor or the control board typically requires handling refrigerant-side components and working inside the outdoor unit, which is best handled by a certified HVAC technician.

If the fix is simply reseating a loose connector, there may be no parts cost, just the service call charge. Replacing a faulty high pressure sensor typically involves parts costs in the range of $30 to $120 depending on the specific sensor, plus labor. If the outdoor unit control board requires replacement, total repair costs including labor can range from $300 to $700 or more.