LG HVAC Error Code 21

Error code 21 indicates the inverter compressor is drawing excessive amperage. The system's inverter drive board detected current above the acceptable operating threshold and shut down the compressor to prevent damage. The cause can range from low supply voltage and incorrect refrigerant charge to a high-head pressure situation or failing compressor windings. A full electrical and refrigerant system diagnosis is required to resolve it.

No. Error code 21 is a protective fault, meaning the system has already shut down the compressor to prevent damage. Attempting to force the system to run while this fault is active risks damaging the inverter drive board and the compressor itself, both of which are expensive components. The unit should remain off until the underlying cause is diagnosed and corrected.

Repair costs vary significantly depending on the root cause. If the issue is a low refrigerant charge, expect to pay for diagnostic labor plus refrigerant, typically in the range of $150 to $400. If the compressor windings have failed and the compressor requires replacement, costs can range from $800 to $2,000 or more depending on the unit size and labor rates in your area. An inverter drive board replacement generally falls somewhere in between, often in the $300 to $900 range including parts and labor.

Some of the preliminary checks, like verifying the circuit breaker, inspecting the outdoor unit for blocked airflow, and confirming the condenser coil is clean, can be done by a homeowner. However, checking supply voltage under load, measuring compressor winding resistance, and verifying refrigerant charge using an electronic scale require proper tools and, in the case of refrigerant work, EPA certification. Most homeowners will need a licensed HVAC technician to complete this diagnosis.

A power reset may temporarily clear the fault and allow the system to restart, but if the underlying cause is not corrected, the error will return quickly once the compressor begins drawing excessive current again. Repeated resets without fixing the root cause can accelerate wear on the compressor and inverter board. A proper diagnostic must be performed to permanently resolve the fault.