AUX stays on constantly
The thermostat shows AUX for hours, utility use jumps, and the house may still feel only moderately warm.
Start here: Check whether the outdoor unit is running at all and whether the air filter is badly clogged.
Heat pump heating problem
Heat Pump Won't Switch Back From Aux
Direct answer: If a heat pump will not switch back from AUX, the usual reason is that the outdoor heat pump side is not carrying the load. Start with thermostat mode, filter condition, airflow, and whether the outdoor unit is actually running before you assume a control failure.
Most likely: The most likely causes are a thermostat setup issue, a badly restricted air filter, an outdoor unit that is not running, or an outdoor coil iced over so the system keeps leaning on backup heat.
A heat pump will use AUX heat during cold weather, defrost, or recovery from a big temperature setback. But if AUX stays on long after the house should be stable, something is off. Reality check: in very cold weather, some systems really do run AUX a lot. Common wrong move: cranking the thermostat up several degrees, which can call for more backup heat and make the problem look worse.
Don’t start with: Do not start by replacing the thermostat or opening electrical compartments. AUX heat can stay on because the heat pump is struggling, not because the thermostat is bad.
If the outdoor unit is silent while the indoor unit heats,check power, ice buildup, and thermostat mode before touching parts.
If AUX stays on only after a big overnight setback,return to a smaller temperature change and see whether the system drops back to normal heat.
Last reviewed: 2026-04-05
What this usually looks like
AUX comes on after you raise the setpoint
The system was normal, then AUX appeared after you bumped the thermostat up several degrees.
Start here: Lower the setpoint closer to room temperature and see whether the system returns to normal heat after it stabilizes.
AUX stays on during cold or icy weather
The outdoor unit may have frost or solid ice on it, and the system seems stuck using backup heat.
Start here: Look for normal light frost versus heavy ice buildup and listen for outdoor unit operation.
Thermostat says AUX but the house is warm
Comfort is mostly okay, but the display keeps showing AUX or seems slow to drop it.
Start here: Check thermostat programming, heat pump system type settings, and whether the thermostat was recently replaced or reset.
Most likely causes
1. Thermostat settings or programming are pushing AUX heat
A recent thermostat change, aggressive recovery schedule, or wrong heat pump setup can keep backup heat energized longer than it should.
Quick check: Set the thermostat to Heat, fan to Auto, cancel large scheduled recovery, and watch whether AUX clears after 15 to 30 minutes.
2. Restricted airflow is making the heat pump fall behind
A packed filter, blocked returns, or closed supply registers can make the indoor coil run colder and reduce delivered heat, so AUX stays in the picture.
Quick check: Pull the filter and inspect it in good light. If it is gray and packed, replace it and reopen blocked vents.
3. The outdoor heat pump unit is not running or not heating well
If the outdoor fan or compressor is off, the system often falls back to electric backup heat to keep the house from dropping.
Quick check: With a heat call active, go outside and listen for the outdoor unit. A quiet unit, humming only, or repeated short starts points to a service issue.
4. The outdoor coil is iced over or the defrost cycle is not clearing it
A little frost is normal. Thick ice on the coil or base pan is not. When the outdoor side cannot exchange heat, AUX may stay on much longer.
Quick check: Look through the grille for a light even frost versus heavy white ice covering much of the coil.
Step-by-step fix
Step 1: Reset the call for heat and rule out a thermostat-driven AUX call
A lot of stuck-on AUX complaints start with a big setpoint jump, a recovery schedule, or a thermostat that is not really in normal heat operation.
- Set the thermostat to Heat, not Emergency Heat.
- Set the fan to Auto, not On.
- Lower the setpoint to 1 to 2 degrees above room temperature instead of several degrees higher.
- If you use schedules, cancel any active recovery period or hold a steady temperature for now.
- Wait 15 to 30 minutes and watch whether AUX disappears once the room temperature gets close to the setpoint.
Next move: If AUX drops out after the smaller call for heat, the system may be reacting normally to a large temperature jump rather than failing. If AUX stays on even with a small heat call, move on to airflow and outdoor unit checks.
What to conclude: This separates normal backup-heat behavior from a system that cannot return to regular heat pump operation.
Stop if:
- The thermostat is blank, flickering, or showing error messages.
- You smell burning plastic or hot electrical odor near the air handler or thermostat.
- The thermostat only works in Emergency Heat or will not control the system normally.
Step 2: Check the filter and basic airflow first
Poor airflow is common, safe to check, and can make a heat pump rely on AUX because the system cannot move enough heat through the house.
- Turn the system off at the thermostat.
- Remove the return air filter and inspect both sides.
- Replace the filter if it is visibly loaded with dust, pet hair, or construction debris.
- Make sure major return grilles are not blocked by furniture, rugs, or boxes.
- Open closed supply registers in the main living areas.
- Turn the system back on and let it run for 20 to 30 minutes.
Next move: If airflow improves and AUX begins cycling off normally, the restriction was likely the main problem. If airflow seems normal but AUX still stays on, check the outdoor unit next.
What to conclude: A heat pump that is starved for airflow often looks like a control problem when it is really a basic maintenance problem.
Stop if:
- The filter slot is wet, the blower area is making scraping noises, or you see soot or scorch marks.
- Airflow is extremely weak at most vents, which can point to a blower or duct problem.
- Opening the cabinet would require removing safety panels beyond the filter access.
Step 3: See whether the outdoor unit is actually running in heat mode
If the outdoor side is down, the indoor backup heat may carry the house by itself and the thermostat will keep showing AUX.
- With the thermostat calling for heat, go outside and listen at the outdoor unit.
- Look for the outdoor fan spinning and listen for the deeper compressor sound, not just a faint hum.
- Check the nearby disconnect and the main breaker panel for a tripped breaker. Reset a breaker once only if it is clearly tripped.
- Clear leaves, snow, or debris that are blocking the outdoor coil or fan guard, using your hands only with power off at the thermostat.
- If the unit starts after a reset, monitor it closely for the next cycle.
Next move: If the outdoor unit starts and stays running, give the system time to recover and see whether AUX drops out on its own. If the outdoor unit stays off, hums without starting, or trips the breaker again, stop there and call for service.
Stop if:
- The breaker trips again after one reset.
- You hear loud buzzing, hard starting, metal clatter, or see burnt wiring.
- The disconnect, whip, or service panel looks damaged or wet.
Step 4: Look for icing and a defrost problem without forcing anything
A little frost is normal in heating season. Heavy ice that does not clear is not, and it can keep the system stuck leaning on AUX heat.
- Inspect the outdoor coil through the grille.
- A thin, even frost on a cold day can be normal. Thick ice across large sections of the coil, fan area, or base pan is not.
- If the unit is heavily iced, turn the thermostat to Off and the fan to Auto. Do not chip or pry ice off the coil.
- Let the ice sit and melt naturally if weather allows, and keep the area around the unit clear so water can drain away.
- After the coil is clear, restart the system and watch whether the unit returns to normal operation or ices up again.
Next move: If the ice clears and does not return, the system may have been temporarily blocked by snow, slush, or poor drainage. If ice returns quickly or never fully clears, the system needs professional diagnosis for defrost, airflow, or refrigerant issues.
Stop if:
- The fan blades are hitting ice.
- Ice is thick enough to deform fins or block the fan opening.
- You suspect refrigerant loss or see oily residue around the outdoor coil or line set.
Step 5: Decide whether this is normal weather behavior, a thermostat issue, or a service call
By this point you have ruled out the easy homeowner fixes and separated normal AUX use from a heat pump that is not carrying its share.
- If AUX only appears during very cold weather or after a large setback, return to smaller temperature changes and steady scheduling.
- If the house is warm but the thermostat display still seems wrong, review whether the thermostat was recently replaced, reset, or misconfigured for a heat pump system.
- If the outdoor unit is not running right, keeps icing, or the breaker trips, book HVAC service instead of replacing parts by guess.
- If airflow is still weak even with a clean filter, follow the low-airflow path at /heat-pump-airflow-weak-in-heat-mode.html.
- If the outdoor unit runs but the air still is not warm enough, follow /heat-pump-air-from-vents-not-warm-enough.html for the next diagnosis.
A good result: If steady settings and a clean filter let the system return to normal heat, keep monitoring for a few days.
If not: If AUX still will not drop out under normal conditions, the remaining causes are usually thermostat setup, defrost/control faults, or outdoor unit performance problems that need testing.
What to conclude: You now know whether this was a usage issue, a maintenance issue, or a true equipment problem.
Stop if:
- You are considering opening the air handler or outdoor electrical compartment.
- The system is using Emergency Heat to keep the house warm.
- You are not sure whether the thermostat is correctly configured for a heat pump with auxiliary heat.
Replacement Parts
Repair Riot may earn a commission from qualifying purchases, at no extra cost to you.
FAQ
Is it normal for a heat pump to stay on AUX in very cold weather?
Sometimes, yes. In colder weather a heat pump may need backup heat longer, especially during morning recovery or defrost. What is not normal is AUX staying on for hours in mild weather or after the house has already reached temperature.
Why did AUX come on after I turned the thermostat up?
A big setpoint jump often tells the system to recover quickly, and many heat pump setups bring on auxiliary heat to help. Try smaller temperature changes and see whether AUX drops out once the room gets close to the target.
Can a dirty filter really keep a heat pump in AUX?
Yes. A badly clogged filter can choke airflow enough that the heat pump falls behind, so the system keeps using backup heat. It is one of the first things worth checking because it is common and safe to fix.
If the outdoor unit is running, can the thermostat still be the problem?
Yes, but do not jump there first. The thermostat can be misconfigured for the wrong system type or can hold AUX too long, especially after replacement or reset. Still, weak outdoor performance, icing, and airflow problems are often more common than a bad thermostat.
What is the difference between AUX heat and Emergency Heat?
AUX heat is backup heat the system adds automatically when the heat pump needs help. Emergency Heat is a manual mode that shuts the heat pump side off and runs backup heat only. If you are relying on Emergency Heat, the system needs service.
Should I reset the breaker if the outdoor unit is not running?
You can reset a clearly tripped breaker once. If it trips again, stop. Repeated trips usually mean a real electrical or mechanical fault, and continuing to reset it can make the damage worse.