Symptoms
- Set to Auto but only heat or only cool ever runs
- Doesn't switch modes even with a big day-to-night temperature swing
- Manual mode switching works fine
Likely causes
- CommonDeadband setting too wideAuto mode uses a temperature gap between heat and cool settings, and a wide gap can make switching feel unresponsive.
- PossibleSystem wiring doesn't support automatic switchingSome systems, especially older ones, need a specific reversing valve or wiring setup for true auto mode.
- PossibleOutdoor temperature lockout settingSome thermostats or heat pumps have settings that prevent cooling below a certain outdoor temperature, or heating above one.
- PossibleThermostat configured for heat-only or cool-only equipmentIf the equipment type is set incorrectly, Auto mode may not have both options available.
What to do
- Check the thermostat's heat and cool set points; if they're set far apart, narrow the gap to see if switching becomes more responsive.
- Confirm your HVAC equipment can physically provide both heating and cooling (not all systems can).
- Check the equipment type setting in the thermostat's configuration menu.
- Look for any outdoor temperature lockout settings that might be preventing a mode from engaging.
- Test each mode manually (Heat only, then Cool only) to confirm both work independently before troubleshooting Auto specifically.
- If manual modes work but Auto never switches appropriately, adjust the deadband or consult the thermostat's Auto mode settings.
Tools and parts
- Thermostat manual
When to call a pro
Call an HVAC technician if your system should support both heating and cooling but Auto mode still won't switch appropriately after checking settings.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.