Nest Thermostat - What does e73 error code mean?

The most common reason for error code e73 / no power to Rc wire detected to appear on the Nest Gen3 display is usually due to AC outdoor unit/compressor shutting down which in turn shuts down power to the Nest thermostat. There are many reasons why the outdoor unit would shut down such as compressor overheating, blocked filters, low gas, issue with the outdoor fan motor or in more serious cases, issues with the compressor itself.  In most cases, the issue is resolved by servicing of the AC outdoor unit. Customers are advised to consult with their maintenance company or call HomeIQ maintenance team.