Google Nest Learning Thermostat (3rd and 4th gen): quick sheet

Covers: Nest Learning Thermostat 3rd generation and 4th generation (released 2024). 24 V systems only.

Single sourceBrands: Google Nest4 components

The best-known smart thermostat. The 4th gen is designed to run with or without a C wire using power sharing from the other wires; Google says fewer than 1% of systems need a C wire or the Nest Power Connector. Most callbacks are power (no C wire) and wiring-label mistakes.

Safety first

Turn off HVAC power before rewiring.
Never connect to line-voltage (120/240 V) baseboard thermostats.

How it works

The thermostat draws power from the HVAC 24 V transformer; without a C wire it shares power through the other control wires.

4th gen supports up to 3 heat stages (W1-W3), 3 cool stages (Y1-Y3) and fan stages G, G2, G3 on most 24 V systems.

4th gen's '*'-style extra terminal accepts only one of W3, Y3, G3, E, HUM or DEHUM; AQ+/AQ- terminals accept Y3 or 1- or 2-wire VENT, HUM or DEHUM.

Works with Nest Temperature Sensors for room averaging.

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Digital thermostat mounted on a concrete wall by Alexander Mass, Pexels License

Diagnostic guides for this appliance

Guide · Thermostat

Indoor fan won't turn off from the thermostat

Start at the thermostat: fan set to ON instead of AUTO, or a setpoint the system can never reach. If the thermostat is right, the cause is in the equipment: a stuck fan relay, a limit switch keeping the blower on, a shorted G wire or a control board fault.

4 steps ~40 minmoderate

Guide · Thermostat

Thermostat shows WAIT or flashing 'Cool On'

Most thermostats hold the compressor off for a few minutes after it stops or after power returns, to prevent short cycling. That delay is normal. If the message lasts well past the delay or the system never starts, check mode, breakers, filter-related lockouts and wiring.

5 steps ~20 mineasy

Guide · Thermostat

Thermostat setpoint changes by itself

Most 'ghost' changes come from features, not faults: a program schedule resuming after a temporary hold, learning schedules, eco/away modes, utility demand-response events, or another person or app. Check the thermostat history before replacing it.

4 steps ~20 mineasy

Other thermostats manuals

Manual · Thermostat

ecobee Smart Thermostat Premium / Enhanced and the Power Extender Kit: quick sheet

ecobee Smart Thermostat Premium and Enhanced (current lineup), plus older ecobee3 lite / ecobee4 that use the same Power Extender Kit (PEK). 24 V AC systems only.

3 components2 common failuresecobee

Manual · Thermostat

24 V HVAC thermostat control wiring: field service manual

Residential 24 VAC forced-air furnaces, air conditioners, air handlers and heat pumps (single and multi-stage). Not for line-voltage or millivolt systems.

5 components4 common failuresAll

Manual · Thermostat

Smart thermostat installation & troubleshooting: field service manual

Wi-Fi thermostats for 24 VAC systems: Nest Learning/Nest Thermostat, ecobee, Honeywell Home T-series/Lyric, Sensi, Amazon Smart Thermostat. Line-voltage models (e.g. for baseboard heat) are covered only by the safety notes.

3 components5 common failuresGoogle Nest, ecobee, Honeywell Home / Resideo, Emerson Sensi, Amazon