The heating and cooling system has experienced a nest thermostat failure resulting in a service code e53 faulty y1 wire. Thermostat is incomplete lockout of all modes. During testing when y1 was removed from the thermostat sub base the thermostat functions return to normal with heating only. Test run multi-stage heating mode through thermostat test mode and temperature increase, Test run cooling mode / y1 wire through hot wires at the furnace control board with the thermostat y1 disconnected at the thermostat. Cooling functions cycled on and cycled off as expected. actual performance testing was not completed today during diagnostics only Recommend homeowner order replacement nest or thermostat of his choice to be installed ASAP at the heating system PMI tune up Equipment is currently running in heating only mode until thermostat is replaced