Nest Thermostat Error E9 — What It Means & How to Fix It

What it means

The auxiliary (backup) heat system isn't responding when called.
On a heat pump, aux heat is what fills the gap when the outdoor temperature gets low enough that the heat pump alone can't keep up — usually electric strips, sometimes a gas furnace.
E9 means the Nest asked for it and got nothing back.
In a cold snap, this is the warning that you may not have enough heat tonight.

Affected Models

Nest Learning Thermostat (1st, 2nd, 3rd gen)
Nest Thermostat E
Google Nest Thermostat (2020)

Common Causes

W (or W2) wire loose at the Nest base or HVAC control board
Heat strip breaker tripped, or thermal limit switch open
Aux gas furnace has its own fault and isn't responding
Heat strip sequencers failed
Aux Heat Lockout temperature set too low in Nest settings

How to Fix It

Check the Aux Heat Lockout setting first.

Settings > Equipment > Continue > Heat Pump Settings > Aux Heat Lockout.
Default sweet spot is around 35°F (2°C).
Set too low and aux never kicks in even when the home is freezing — free fix if this is the issue.
Power off and reseat the W wire.

Pull the Nest off, push the W (and W2 if present) firmly into the terminal, snap it back on.
Loose W wire = aux never gets the on signal even though the relay is fine.
Find and check the heat strip breaker.

Usually a separate double-pole 240V breaker labelled 'Air Handler', 'Electric Heat', or 'Heat Strips' in the panel.
Reset it once if tripped.
If it trips again when aux runs, there's a real fault in the strips themselves and the breaker is doing its job.
Try Emergency Heat as a temporary workaround.

Emergency Heat skips the heat pump entirely and runs straight on aux.
If Emergency Heat works but normal Heat doesn't, the heat-strips themselves are fine — the issue is in the mode-switching logic.
Emergency Heat is much more expensive to run, so don't leave it on long.
Call HVAC if the above didn't fix it.

Tell them whether Emergency Heat works.
That single fact tells the technician where to start: Emergency Heat working narrows it to wiring or sequencers; Emergency Heat failing means the strips themselves.
On systems past 10 years, failed heat strips are routine.

When to Call a Professional

Heat strips run on 240V and aren't user-serviceable.
If the breaker keeps tripping or strips clearly aren't firing, you need a licensed HVAC technician or electrician.
In genuinely cold weather this is an emergency — get someone the same day.

Frequently Asked Questions

What is auxiliary heat and when does the Nest use it?

Aux heat is the backup that fills in when the heat pump can't keep the house warm on its own.
Heat pumps work by pulling heat from outside air — easy at 50°F, much harder at 20°F.
Below about -1 to 4°C (30–40°F depending on the system), the heat pump runs out of capacity and the Nest automatically calls for aux: electric strips, or a gas furnace if you have a dual-fuel setup.

My Nest shows AUX or AUX HEAT instead of HEAT — is that the error?

No.
AUX HEAT just means the aux system is currently running, which is normal in cold weather.
E9 is the error code — different display.
If you only see AUX HEAT in the regular heat indicator, your system is doing exactly what it's designed to do.

Heat pump works fine but aux heat fails — what's that mean?

It means the heat pump is healthy and the issue is contained to the aux side.
Check the heat-strip breaker, the W wire, and the lockout setting.
If all those check out, the strips, sequencers, or wiring need a technician.
In the meantime, set the thermostat higher before cold weather hits — heat pump alone can keep up if the home is already warm.