On the covered Google Nest thermostat, E2, E22, E72, E402 means the thermostat detects no power wires. Google says an Rc or Rh wire is required. First, check that the heating and cooling system has power at its switch and breaker, then look at the Rc and Rh connectors on the thermostat base.
Try in this order
0 of 4 completedWhat to do first
ReversibleCheck that the heating and cooling system has power at its switch and breaker, then look at the Rc and Rh connectors on the thermostat base.
Correct only the documented fault domain
ReversibleGoogle's "Troubleshoot power" steps: first make sure the system's power switch is on (there may be one switch for heating and one for cooling, and both must be on), then turn power off, pull the display and check that every wire is fully inserted with its connector button down and about 3/8 inch (6 mm) of bare wire showing, and that each wire is connected properly at the system board. If it still shows the code, Google sends you to a local professional.
Check the Rc and Rh wires at the base
ReversibleGoogle says Rc or Rh must be connected for the thermostat to work. With the breaker off, detach the display and confirm the wire is fully inserted in its terminal.
Test for 24 volts
ReversibleWith power back on, measure across the Rc wire and the common terminal; about 24 V AC means the wiring is fine and the fault is upstream in the HVAC system.
Evidence ledger
Sources behind this answer
Exact questions
Common questions about “Google Nest thermostat code E2, E22, E72, E402”
What does E2, E22, E72, E402 mean on a Google Nest thermostat?+
The thermostat detects no power wires. Google says an Rc or Rh wire is required.
Does a Nest thermostat need a C wire?+
It needs Rc or Rh for power. If the system has no common wire, a Nest Power Connector can supply the missing path.
What is the difference between E2 and E3 on a Nest thermostat?+
E2 type codes mean no power wires were detected; E3 type codes mean the Rc wire is connected but carries no power.
Can I fix a Nest E72 myself?+
If you are comfortable switching off the breaker and reseating a wire, yes. Anything involving the furnace control board or a missing C wire is the job for an HVAC technician or a Nest Power Connector.
Does a Nest thermostat work without an Rc wire?+
It needs either Rc or Rh to get power, so without one of them it will not run.
My Nest shows E2 after I swapped my old thermostat. Which wires should I check?+
Compare the photo of the old wiring with what is in each terminal on the new base. A wire that is pushed in but not clamped, or tucked into the wrong letter, is the commonest slip after a swap.
Is a C wire the same as Rc or Rh?+
No. The Rc and Rh terminals bring 24 volt power from the transformer, while C is a return path. Google's message for this code is about Rc or Rh, so check those two first.
Why does the display work for a while and then show E2?+
A wire that makes poor contact can carry enough current to start the display and drop out as the battery drains, so the code appears hours later.