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

What Does This Error Mean?

Nest error E9 means there is a fault with the auxiliary (backup) heat system. Heat pumps use auxiliary heat — electric resistance strips or a gas furnace — when it is too cold outside for the heat pump to work efficiently. When E9 appears, the Nest tried to activate auxiliary heat and the system did not respond correctly. This can leave you without adequate heating in cold weather.

Affected Models

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

Common Causes

The W (auxiliary heat) wire is loose or disconnected at the Nest terminal or at the HVAC control board
The electric heat strips in the air handler have a blown breaker or a thermal limit switch that has tripped
The auxiliary gas furnace has its own fault and is not responding to the Nest's heat request
The sequencers that activate the electric heat strips in stages have failed
The Nest settings have auxiliary heat configured incorrectly — such as a wrong lockout temperature

How to Fix It

Check the auxiliary heat lockout temperature setting on the Nest. Open the Nest app, go to Settings > Equipment > Continue > Heat Pump Settings. Find the 'Aux Heat Lockout' setting. If it is set to a temperature that is too low, the Nest may not be activating auxiliary heat when it should. The typical setting is 35°F (2°C).

If the lockout temperature is set too low, the Nest will not run auxiliary heat until outdoor temperatures drop very low — which can mean the home never gets warm enough on a cold day. Adjusting this setting is a free fix worth checking first.
Turn off HVAC power and check the W or W2 wire (auxiliary heat wire) at the Nest base. Pull the Nest off the base and confirm the wire is fully seated and locked in its terminal. For systems with both W1 (primary heat) and W2 (second stage/aux heat), verify both are correctly connected.

On heat pump systems with a Nest, the W wire controls auxiliary heat. A loose W wire means the auxiliary heat never receives the startup signal.
Check the circuit breaker for the auxiliary heat strips. Electric heat strips in an air handler typically have their own 240V circuit with a dedicated double-pole breaker. If this breaker has tripped, reset it. If it trips again when auxiliary heat runs, there is a fault in the heat strips requiring a technician.

The auxiliary heat breaker may be separate from the main HVAC breaker. Look for it labeled 'Air Handler,' 'Electric Heat,' or 'Heat Strips' in your electrical panel.
Set the Nest to Emergency Heat mode as a temporary measure. In the Nest app or on the thermostat, switch to Emergency Heat mode (Emrg Heat). This bypasses the heat pump and runs only the auxiliary heat directly. If heating works in Emergency mode but not normal mode, the issue is the mode-switching logic rather than the auxiliary heat itself.

Emergency Heat mode uses more electricity than normal heat pump operation, so it is more expensive to run. Use it only as a temporary workaround while you arrange a repair.
Call an HVAC technician to diagnose the auxiliary heat fault. Tell them which steps you have already tried and whether Emergency Heat mode works. They will test the heat strips, sequencers, circuit breakers, and thermal limit switches to find the specific failure.

Electric heat strips and their associated components (sequencers, thermal limits, fuses) have a finite lifespan. On systems more than 10 years old, failed heat strips are common and usually require replacement.

When to Call a Professional

Electric heat strip failures and gas furnace faults in the auxiliary heating system require a professional. Electric heat strips run at high voltage (240V) and should only be inspected and repaired by a licensed HVAC technician or electrician. In very cold weather, a failed auxiliary heat system is an emergency.

Frequently Asked Questions

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

Auxiliary heat (also called backup heat or aux heat) is the secondary heating system that supports the heat pump when it cannot provide enough warmth on its own. Heat pumps work by extracting heat from outdoor air. In very cold weather (below about 30 to 40°F / -1 to 4°C depending on the system), there is not much heat to extract and the heat pump struggles to warm the home. The Nest automatically activates auxiliary heat when the heat pump alone cannot maintain the set temperature. Auxiliary heat is usually electric resistance strips or a gas furnace.

My Nest shows AUX or AUX HEAT instead of HEAT. Is that different from E9?

AUX HEAT displayed on the Nest is normal — it just means auxiliary heat is currently running. This is not an error. E9 is an error that appears when the Nest tried to activate auxiliary heat and something went wrong. Seeing AUX HEAT on a cold day is expected behavior for a heat pump system in a cold climate. Only worry if the display shows the E9 error code specifically.

My heating works in heat pump mode but fails when auxiliary heat should kick in. What does that mean?

This clearly isolates the problem to the auxiliary heat system — the heat pump itself is working correctly. Check the circuit breaker for the heat strips, the W wire connection at the Nest, and the auxiliary heat lockout temperature setting. If those are correct, the heat strips, sequencers, or their control wiring have a fault that needs a technician. In the meantime, you can raise your home's temperature aggressively in heat pump mode before outdoor temperatures drop to the point where auxiliary heat is needed.