ecobee Auxiliary Heat Not Working — What It Means & How to Fix It

What Does This Error Mean?

Your ecobee is not activating auxiliary (backup) heat when it should. Aux heat normally kicks in when outdoor temperatures are very low and the heat pump alone cannot keep up. This is usually caused by a wiring issue, incorrect equipment settings, or a problem with the heat pump itself.

Affected Models

ecobee SmartThermostat Premium
ecobee SmartThermostat Enhanced
ecobee3 lite
ecobee4
ecobee SmartSensor

Common Causes

The aux heat threshold is set too low — aux heat only activates when it's extremely cold outside
The AUX or E terminal wiring is loose, disconnected, or wired incorrectly
The heat pump is not functioning, so it cannot trigger the aux stage
The ecobee equipment settings do not match your actual heating system type
A safety lockout on the air handler or furnace is preventing aux heat from running

How to Fix It

On ecobee, go to Main Menu > Settings > Threshold > Aux Heat Threshold and check the setting.

If the threshold is set to -20F, aux heat won't activate until it's that cold outside. Raise it to a more reasonable temperature like 35F or 40F.
Go to Main Menu > Settings > Equipment > Heat Pump and confirm that 'Aux Heat' is enabled.

If aux heat is toggled off in the equipment settings, the thermostat will never send a signal to activate it.
Turn off power to the system at the breaker, then check the AUX and E terminal wiring at the ecobee base.

Aux heat typically uses the AUX or W2 terminal. A loose or missing wire here will prevent aux heat from activating.
Test the heat pump itself by setting the system to Heat mode and raising the temperature — confirm warm air is coming out.

If the heat pump is not working at all, the aux stage may never trigger. Fix the heat pump issue first.
On ecobee, go to Main Menu > Settings > Equipment and verify the system type is set to 'Heat Pump' with auxiliary heat.

If it is set to 'Conventional', ecobee will not know to use a heat pump + aux configuration.
Restart the ecobee by going to Main Menu > Settings > Reset > Restart Thermostat, then retest.

A soft restart clears temporary faults and re-initializes equipment communication.

When to Call a Professional

If the aux heat still won't activate after checking all settings and wiring, call an HVAC technician. Problems with the heat pump refrigerant level, reversing valve, or air handler wiring require a professional.

Frequently Asked Questions

What is the difference between aux heat and emergency heat on ecobee?

Aux heat activates automatically alongside the heat pump when it gets too cold for the heat pump alone. Emergency heat bypasses the heat pump entirely and runs only the backup heat source. You should only use emergency heat if the heat pump is broken — it is much more expensive to run.

How cold does it have to be for ecobee aux heat to turn on?

By default, ecobee activates aux heat based on the 'Aux Heat Threshold' setting in your preferences. You can adjust this in Main Menu > Settings > Threshold. A common setting is 35F to 40F outdoor temperature.

Why does my ecobee say 'Aux On' but I feel no extra heat?

ecobee may be sending the signal but the air handler or furnace is not responding. Check the air handler for error lights, a tripped breaker, or a clogged filter causing a safety shutoff. If the filter is extremely dirty, replace it and wait 10 minutes for the system to reset.