DIAGNOSIS The thermostat detected an invalid wiring configuration during its power-up self-test.
FIX The error locks the device. You must do a full reset. Press and hold the Menu button for 10 seconds until the screen clears. This resets to factory defaults. You will need to reconfigure everything from scratch.
IF THAT DIDN'T WORK The error is likely real. The most common trigger is having both the O/B wire (heat pump reversing valve) and W wire (aux heat) connected in a standard furnace configuration. You can only use one or the other based on your system type in the menu.
HEADS UP Document your original thermostat's wiring with a photo before removing it. This error often stems from misidentifying the system type during initial setup.