Nest Thermostat Error E31 — What It Means & How to Fix It

What Does This Error Mean?

Nest error E31 means the thermostat detected a problem with the W2 or AUX wire — the wire that controls your secondary heating stage or auxiliary (backup) heat. In a heat pump system, W2/AUX activates backup heating when it is too cold outside for the heat pump to work efficiently alone. In a two-stage furnace, W2 activates the second, more powerful heating stage. When Nest sees an unexpected signal on this wire, it shows E31 and may disable the second heating stage until the issue is resolved.

Affected Models

Nest Learning Thermostat (2nd, 3rd gen)
Nest Thermostat E
Google Nest Thermostat (2020)

Common Causes

The W2 or AUX wire is loose or not fully inserted in the Nest terminal connector
The W2/AUX wire was placed in the wrong terminal during Nest installation
A short circuit — where the W2 wire is touching another wire — is creating a false signal on the terminal
The secondary heating stage or auxiliary heating element in the HVAC system has failed and is not responding to the W2 signal
The W2/AUX setting in the Nest configuration does not match the actual equipment connected

How to Fix It

Turn off all HVAC power at the circuit breaker before touching any wiring. In a heat pump system with electric backup heat, high-voltage components are present — always cut power first.

Locate and switch off both the air handler breaker and the outdoor unit breaker in your electrical panel before working on the thermostat wiring.
Pull the Nest off its base and locate the W2 or AUX terminal. Press the orange release button, remove the wire, and firmly reinsert it until it seats securely. Confirm the terminal button is fully released and the wire is locked in.

A slightly loose W2 wire is the most common cause of E31. The wire should not come out without pressing the release button.
Confirm the W2 wire is in the correct terminal. The wire should be in the terminal labeled W2 or AUX — not W1, Y, or another nearby terminal. Compare with your previous thermostat's wiring or a photo you took during installation.

Nest's wiring diagram in the app (Settings > Equipment) shows which wires should be where. This is a useful reference if you are unsure whether the wire is in the right place.
Check the Nest equipment settings to confirm W2/AUX is configured correctly. Go to Settings > Equipment > Continue and verify the W2 setting matches your system — either a second-stage heat for a furnace or auxiliary/emergency heat for a heat pump.

Selecting the wrong equipment type for the W2 wire will cause Nest to send the wrong signals and trigger E31.
Restore power and test the second heating stage. Set the temperature well above room temperature and watch the system. If you have a heat pump, wait for the Nest to call for auxiliary heat (usually when outside temperatures drop significantly or you raise the setpoint by 3+ degrees).

You can check whether auxiliary heat is running in the Nest app under Home > current heat status. If AUX heat is shown as running but you hear nothing from the system, the auxiliary heating equipment itself may be faulty.

When to Call a Professional

If the W2 wiring is correct and the error persists, call an HVAC technician. The auxiliary or second-stage heating system may have a failed component — such as a burned heating element, a failed sequencer, or a problem in the heat pump's control board. In very cold weather, losing auxiliary heat can mean the heat pump alone cannot keep your home warm. Treat this as urgent if temperatures are near or below freezing.

Frequently Asked Questions

What is auxiliary heat and when does it kick in?

Auxiliary heat is a backup heating system that works alongside a heat pump. Heat pumps are efficient but lose effectiveness as outdoor temperatures drop below freezing. When it is very cold outside, the heat pump alone cannot extract enough heat from the air to warm your home. At that point, the auxiliary heat kicks in — usually electric resistance heating strips inside the air handler. The Nest automatically activates auxiliary heat when the heat pump needs help or cannot keep up with demand.

Will losing auxiliary heat damage my heat pump?

The heat pump itself will not be damaged by auxiliary heat being unavailable. However, your home will be harder to heat in very cold weather because the heat pump alone may not be powerful enough. The heat pump will run for longer and work harder trying to reach the set temperature. In extreme cold, it may not reach the target temperature at all. This is why E31 should be addressed promptly in cold climates.

What is a two-stage furnace and does E31 affect it?

A two-stage furnace has two firing levels: a lower first stage that handles mild days efficiently, and a higher second stage for very cold days. The W2 wire activates the second stage. If E31 blocks the W2 signal, your furnace will only run on its first, lower stage. This means it may struggle to heat your home on cold days and will run for longer — increasing your energy bill. The furnace itself will not be harmed, but comfort and efficiency will both suffer.