Symptoms
- Mode selector shows Heat and won't change
- Furnace runs even when set to Cool or Off
- App shows a different mode than the thermostat displays
Likely causes
- CommonSoftware or app sync glitchThe physical thermostat and its app can occasionally get out of sync on mode.
- PossibleRestricted mode setting enabledSome thermostats let installers lock out cooling, often in heating-only climates.
- PossibleStuck heat relay in the furnace or heat pumpA mechanical relay can occasionally stick in the closed position.
- Less commonWiring issue crossing heat and cool circuitsIncorrect wiring can make the system respond to heat calls regardless of mode.
What to do
- Try switching modes directly on the thermostat rather than through the app, and vice versa, to rule out a sync issue.
- Restart the thermostat by removing it from its base or pulling batteries for 30 seconds.
- Check the installer or advanced settings menu for a locked or restricted mode setting.
- Turn off system power at the breaker for one minute to reset any stuck relay, then restore power and retest.
- If the furnace still runs regardless of mode after a full power reset, the relay or wiring likely needs professional attention.
Tools and parts
- Screwdriver
When to call a pro
Call an HVAC technician if a full power cycle doesn't clear the stuck mode, since it may be a failing relay inside the equipment.
Safety
Use caution
Turn off the breaker before resetting power to the furnace or air handler for safety.