Fix it
Smart Thermostat Not Heating or Cooling (2027 Fix Guide)
Fix a smart thermostat that will not heat or cool: wiring, power, and settings checks in order.
Most smart thermostat failures trace to three causes: wiring mistakes during install, missing C-wire power issues, or settings mismatches with the HVAC type. Work through these checks before calling a technician; the fix is usually in this list.
Thermostat has no power or keeps rebooting
No C-wire is the prime suspect: power-stealing reboots the unit during heat or cool calls. Verify the C connection at both the thermostat and the furnace board. If no C-wire exists, install the power extender kit or have a wire run. Also check the furnace breaker and the condensate overflow switch, which cuts power to the thermostat when the drain pan fills.
Heat runs when it should cool (or reverse)
The O/B reversing valve setting is wrong for your heat pump. In the installer settings, toggle the O/B orientation: most systems energize on cool (O), some on heat (B). One flip fixes it. If you photographed the old wiring, confirm the O/B wire landed on the correct terminal.
Aux heat runs constantly and bills spike
Check the aux heat lockout threshold: set it near your system balance point so resistance heat only assists below the temperature where the compressor cannot keep up. Verify the thermostat knows it controls a heat pump, not a furnace, in equipment settings. A misconfigured system type is the most common cause of runaway aux.
Room never reaches the set temperature
First rule out HVAC issues: dirty filter, blocked vents, or low refrigerant mimic thermostat failure. Then check that the thermostat is not in a draft, in direct sun, or above a heat source, all of which corrupt its reading. If one room lags, a remote sensor (Ecobee, Honeywell) measuring that room usually closes the gap.
Questions, answered
Why did it work for months then fail?
Seasonal change is the usual trigger: a power-stealing install that survived mild weather fails on the first long heat or cool call. Also check for tripped furnace breakers and full condensate pans, both seasonal.
Should I factory reset?
Only after wiring and settings checks. A reset erases learned schedules and custom thresholds, and the underlying wiring issue will still be there. Re-verify the equipment configuration first.
When do I call an HVAC tech?
Call when the furnace or outdoor unit itself misbehaves (no fan, no compressor, burning smells), when wiring labels do not match any diagram, or when high-voltage equipment is involved. Thermostat settings are DIY; equipment faults are not.