LG Heat Pump Error Code E166

E166 indicates that during the system's automated piping check routine, the EVAP IN (evaporator inlet) temperature changed on an indoor unit other than the one being tested. This points to a mismatch between the physical refrigerant piping and the expected system configuration. It is most commonly caused by crossed refrigerant lines or incorrect indoor unit address assignments.

E166 is not an immediate safety hazard. It reflects a configuration or installation error detected during a diagnostic routine rather than a component failure or refrigerant leak. The system flags the issue to prevent improper operation, so the unit may not function correctly until the fault is resolved.

If the fix only involves reconfiguring address assignments, labor costs are typically in the range of $75 to $200 depending on the technician's rates and time required. If refrigerant lines need to be physically rerouted or a faulty EVAP IN sensor needs replacement, costs can range from $200 to $600 or more, depending on system complexity and parts involved.

Addressing assignment errors can sometimes be corrected by a knowledgeable installer or DIYer with access to the system's configuration interface. However, if the refrigerant piping is physically crossed, correcting it requires a licensed HVAC technician who can safely handle refrigerant. It is recommended to have a professional confirm both piping layout and address settings before re-running the piping check.

A simple power reset will not permanently clear E166 because the error is generated during the piping check routine, which runs as part of commissioning or re-commissioning. The fault will reappear when the piping check is run again unless the underlying piping or address assignment issue is corrected first.