On supported Nest thermostats, E73 is a power-related help code associated with no detected power at Rc.
What E73 means
On supported Nest thermostats, E73 is a power-related help code associated with no detected power at Rc.
What you may notice
- The thermostat shows E73
- Cooling will not start
- The battery may run down
- The thermostat may go offline
Most likely causes
- 01The HVAC breaker or service switch is off
- 02A condensate safety switch opened
- 03The Rc wire is loose
- 04A transformer or control board failed
- 05Recent service changed wiring
Safe checks, in order
Start with the simplest non-invasive check. Stop as soon as the error clears; repeated resets can hide a condition without correcting it.
Check the HVAC breaker
Confirm the heating and cooling system breakers and service switch are on.
Look for a system shutdown
Check for an obvious condensate overflow or equipment fault without opening panels.
Reseat the display
Confirm the thermostat display is fully seated on its base.
Call an HVAC professional
Use a pro if power does not return or wiring must be tested.
Know where to stop
Turn HVAC power off at the breaker before touching thermostat wires. Do not bypass a condensate switch or open live equipment panels.
When to call a technician or support
Call an HVAC professional promptly during extreme heat or if E73 returns after verified system power.
When you call, have the exact product or vehicle model, model year where relevant, the complete code or symbol, and when it appeared.
Where this information came from
We use the manufacturer's current published guidance as the starting point, then organize it into a safer, beginner-friendly sequence.
Google Nest Help — thermostat power help codes Source last checked August 16, 2026. Manufacturer instructions and model manuals take priority over this independent summary.