On the covered Google Nest thermostat, E8, E28, E78, E408 means additional wires are required because a W2/AUX wire was detected by itself. First, note which wires are in the thermostat base, looking for a W2/AUX wire without the wires it normally comes with.
Try in this order
0 of 7 completedWhat to do first
ReversibleNote which wires are in the thermostat base, looking for a W2/AUX wire without the wires it normally comes with.
Correct only the documented fault domain
ReversibleGoogle's wiring steps: compare the wires with the wiring diagram from setup and correct any wire in the wrong connector, so the thermostat sees a valid set of wires instead of a W2/AUX wire on its own.
Find the W2 or AUX wire
ReversibleThese codes mean the thermostat sees a W2 or AUX wire without the other wires it needs. Check the wire labeled W2 or AUX at the base and the W1 and R wires that should accompany it.
Switch off the power first
ReversibleTurn the heating and cooling system off at its breaker or switch before you take the thermostat off its base, so no wire can short.
Photograph the wires and labels
ReversibleTake a clear photo of the base with the wires in place, with the letters beside each terminal visible, before you change anything.
Compare with the setup diagram
ReversibleCheck the photo against the wiring diagram the Nest app made at setup. A W2 or AUX wire is only valid alongside the wires that run the main heating, so a W2 on its own is the mistake the code is reporting.
Correct only the wire named
ReversibleMove or remove just the wire the diagram shows is wrong, press it firmly into its terminal and restore the power.
Evidence ledger
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Exact questions
Common questions about “Google Nest thermostat code E8, E28, E78, E408”
What does E8, E28, E78, E408 mean on a Google Nest thermostat?+
Additional wires are required because a W2/AUX wire was detected by itself.
Is W2 the same as AUX on a Nest thermostat?+
Nest treats a W2 or AUX wire as the second-stage heat connection, so both labels produce this code when they are connected without the wires that should accompany them.
Why does my Nest need extra wires for W2 or AUX?+
An AUX or W2 connection only works alongside the wires that run the main heat, so a thermostat wired with W2 alone is missing the pieces the heating system needs. Compare the wires in the base with the wiring diagram you made at setup.
Should I take a photo of the wires before changing anything?+
Yes. Switch off the power at the breaker, take a clear photo of the base wiring and the labels, and then compare it with the diagram before moving a wire.
Which wire is W2 on a Nest thermostat?+
W2 is the second stage of heat, often used for auxiliary heat on a heat pump. It is shown on the base next to the W1 or heat wire, and it normally works only with the main heating wires in place.
Do I need an electrician to fix a wiring code?+
Not if the system is off at the breaker and you are only moving a low-voltage thermostat wire back to its correct terminal. If the wiring looks damaged or you are unsure which terminal is right, a heating technician can do it quickly.