First, pin down which problem you actually have, because three different complaints all get called "resetting." The screen goes blank, then the thermostat reboots — that's a power interruption. Your schedule or settings keep reverting to defaults — that's the thermostat losing its memory power, usually batteries. And the setpoint "changes by itself" — that one is most often not a reset at all. It's the programmed schedule doing its job, or a temporary hold quietly expiring.
Replace the batteries — even if it looks wired. Plenty of wall thermostats run on batteries alone, and many wired models keep batteries as backup. A low-battery icon is easy to miss for months. Weak batteries cause exactly the behavior people blame on the thermostat itself: lost settings, blank screens, random reboots. Fresh batteries first, judgement second.
Rule out the schedule. Set a temperature and press Hold. If the setpoint stays put for a full day, nothing was resetting — the program was changing it on schedule, or an earlier hold expired. Half of "my thermostat has a mind of its own" cases end right here.
Check the wires at the wall plate. Pull the thermostat straight off its base and look at the small screw terminals. A wire that's loose or barely clamped — common after the thermostat got bumped, painted around, or installed in a hurry — drops contact when the system vibrates, and the thermostat reboots. Reseat any loose wire snugly in its terminal. The wiring at this end is low-voltage, the same 24-volt setup covered in any basic thermostat install.
Check the condensate float switch. Find the indoor air handler — closet, attic, or basement. If the drain pan or drain line is holding water, the float switch has tripped and cut power to stop an overflow, and it will keep "resetting" your thermostat until the drain is cleared. Fix the clog, don't fight the switch: unclogging the AC drain line is a homeowner-level job and usually the real repair.
Look at the furnace filter. In heating season, a completely clogged filter overheats the furnace, the high-limit switch cuts the burner, and power to the thermostat can drop along with it. Everything cools down, restarts, and repeats — which feels exactly like a resetting thermostat. If your filter looks like a gray rug, replace it; the filter replacement and MERV guide covers sizing and timing.
Check the furnace switch and breaker. Most furnaces have a service switch that looks like an ordinary light switch on or near the unit. It gets bumped during storage runs more often than you'd think, and a half-thrown switch or a tripped breaker makes a thermostat flicker in and out instead of dying outright.
Smart thermostats installed without a common (C) wire "borrow" power by sipping small amounts of current through the heating and cooling circuits. It works until it doesn't. When the system runs hard, the thermostat uses power faster than it can sip it back, the internal battery sags, the screen dies mid-cycle, and the unit reboots — resetting that comes and goes with the weather. If yours blanks out mainly during long heating or cooling runs and there's no wire landed on the C terminal, that's almost certainly your answer, and running a thermostat without a C-wire is its own whole topic, adapters and workarounds included.
Replacing the thermostat first. The thermostat is usually the messenger, not the problem. A brand-new one on the same loose wire, clogged drain, or missing C-wire will reset too, and now you've spent money to learn what the batteries would have told you for free.
Ignoring the low-battery warning because the screen still lights up. The display is the last thing to go. Settings memory gives up weeks before the screen does, so a thermostat can light up cheerfully every morning while quietly forgetting everything you programmed.
Fighting the schedule one day at a time. Manually correcting the temperature every morning and evening, for months, instead of spending five minutes fixing the program or using Hold. If step 2 above cleared the hardware, the program is the thermostat working exactly as told.
Follow the power. Batteries, wall wires, float switch, filter, furnace switch — nearly every common cause sits at the thermostat or one step away from it, and none of them needs a part more exotic than AA batteries or a filter. If all of those check out and the thermostat still loses power and reboots, the likely fault is the furnace control board or transformer. That's a fair HVAC call — and telling the tech what you've already ruled out means you pay for a diagnosis, not a scavenger hunt.
Intermittent power. The usual suspects are weak batteries, a loose wire at the wall plate, a smart thermostat without a C-wire starving during long system runs, or a float switch cutting power when the condensate drain fills. Work through them in that order and you'll find most cases in the first two.
Usually not, if the thermostat has batteries — they carry the memory through an outage. Battery-free models and older thermostats can lose their schedule when power drops. If your settings vanish after every flicker of the lights, check the batteries before you blame the outage.
Often around a year, sometimes less in a hard-working smart thermostat. The practical move is to replace them when the low-battery warning appears — or once a year alongside the smoke detector batteries, so the job never gets forgotten. Weak batteries cause more "broken thermostat" calls than broken thermostats do.
If the reboots happen mainly during long heating or cooling runs, and there's no wire on the C terminal, then yes — that's the classic no-C-wire pattern. Power-stealing works fine at idle and starves under load. An adapter kit or a new wire run solves it permanently.
Two main reasons. On cooling calls, a clogging condensate drain lifts the float switch and cuts power mid-cycle. On smart thermostats without a C-wire, long AC runs drain the battery faster than it recharges. Check the drain pan for standing water first — it's the faster test, and it protects your ceiling either way.