LG Heat Pump Error Code P721

P721 indicates that the outdoor unit's compressor sump temperature reached 95°C and stayed there for at least 5 minutes, triggering the SUMP DOWN 1 protection control. The unit shuts down automatically to prevent compressor damage. The fault points to issues with compressor loading behavior, the PWM valve, or the sump temperature sensor. A technician should use S-net diagnostics to investigate the root cause before restarting the system.

No. The P721 fault represents a protective shutdown triggered by dangerously high sump temperatures. Continuing to operate the unit or bypassing the fault without resolving the underlying cause can result in compressor failure, which is one of the most expensive repairs possible on a heat pump system. Leave the unit off until the fault has been properly diagnosed and corrected.

A power reset may temporarily clear the P721 fault and allow the unit to restart, but the code will return if the underlying condition causing the sump to overheat is still present. Because this fault involves a sustained temperature threshold, the thermal issue must be resolved to prevent repeat shutdowns. Repeated resets without repair increase the risk of permanent compressor damage, so a reset should only be used as part of a supervised diagnostic process.

Some preliminary checks, such as clearing debris from around the outdoor unit or visually inspecting the temperature sensor for obvious looseness, can be done without specialized tools. However, diagnosing the PWM valve and compressor loading behavior requires S-net software access and refrigerant handling knowledge that a certified HVAC technician should perform. Attempting to bypass or ignore this fault risks permanent compressor damage.

Repair costs vary widely depending on the root cause. If the fix is a reattached temperature sensor or a refrigerant adjustment, you may be looking at a service call fee in the range of $100 to $300. A failed PWM valve replacement could run $300 to $700 including labor. If the compressor has been damaged from sustained overheating, replacement costs can range from $1,200 to $3,000 or more depending on the unit size.