Thanks for responding back to us — let's sort this out.
It appears that your system may not provide enough power or any power to either the thermostat or a specific wire (so the thermostat can’t complete a circuit and shows the E73 code). To isolate:
Please note: turn off the power from the HVAC system at the breaker box before touching the wires.
Keep me posted on how it goes.
have any concerns until the temps dropped. This is when I found that the aux heat was not working. With the wiring shown in the pic has worked well for heat pump heat and cooling, just not when there’s a need for the aux heat. Thanks for your continued support.