Why is my thermostat not working?

Wondering what's going on in there?

How your thermostat works, in plain English

Your thermostat is a switch with a brain. It doesn't heat or cool anything. It tells your furnace or AC when to run.

Each number matches a step below. A problem at any step can look like a broken thermostat.

Power. The thermostat runs on batteries, or on low power from the furnace through a wire the pros call the C-wire. No power, blank screen.
The call. When the room is off from your setting, it sends a signal down thin wires. You may hear a click.
The furnace or air handler. It starts only if its power is on, its front panel is shut tight, and its drain isn't backed up. Each has a safety switch.
Heat or cooling. The furnace heats, or the AC outside starts. Honeywell says many systems wait about 5 minutes after a change, to protect the AC's compressor.

So a thermostat that "isn't working" is usually one of these:

No power: batteries, a switch, a breaker, the furnace panel, or a backed-up drain.
A setting: the mode, a schedule, a hold, or the fan.
The thermostat itself, its wiring, or a missing C-wire.
The furnace or AC, not the thermostat.

The questions at the top check these in the same order. Thermostat fine, but no heat? See a furnace that won't turn on or a furnace blowing cold air. No cooling? See an AC blowing warm air.

AC won't start at all? See our guide to an AC that won't turn on.

Who to call

Rather not do it yourself? Call an HVAC tech.

That's a fine choice. Maybe you're busy. Maybe you're selling the house and just want it done right. An HVAC tech (a heating and cooling pro) can fix the wiring or put in a new thermostat.

$60–$200Fix thermostat wiring

$85–$600New thermostat, put in

Most companies charge $75–$250 to come out, and many put that toward the repair.

Find HVAC repair near you

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What to say

Calling a pro? Here's what to say.

A few lines to read when they pick up, and the questions to ask before anyone comes out.

Before you call

Under 10 years old? Call your installer first.

Home warranty? Call them first.

4.5 stars or more, and a license number.

Say this

Hi. My thermostat isn't working. When's the soonest you can get here? And what's your fee for the visit? Does it go toward the repair?

Always ask

What's your fee to come out, and does it go toward the repair?
Will you tell me the full price before you start any work?
Are you licensed and insured?

Watch out if

They push a whole new system on the first visit for a small problem.
They won't give you a price before starting.
They want to "just add refrigerant" every year without finding the leak.

Feeling pushed to decide on the spot? You can say no and hang up. You owe nothing for a phone call.

Know what's wrong before they come

Answer up to 5 quick questions. We'll give you the words a pro uses.

Answer the questions →

More of a list person?

Common thermostat problems, causes, tests and costs

Find the row that sounds like yours. The quick test is something you can check yourself in a few minutes.

What you notice	What it probably is	Quick test	Do it yourself	With a pro	Verdict
Screen blank	The thermostat's batteries are dead	Pop the thermostat off and look for batteries	$0–$10	Not needed	FIXSee the answer: The thermostat's batteries are dead
Furnace switch off, or the furnace or AC breaker tripped	The system's power was off	Look at the furnace switch and the breaker box	Free	Not needed	FIXSee the answer: The system's power was off
Furnace front panel loose, crooked or off	The furnace door switch	Press the front panel. Is it seated all the way?	Free	Not needed	FIXSee the answer: The furnace door switch
Water in the pan under the indoor unit, or around the furnace	A clogged drain shut the system off	Look for water in the pan under the indoor unit	$0–$30	$75–$250	FIXSee the answer: A clogged drain shut the system off
Still blank after batteries, switch, breaker and panel checks	A wiring, fuse or control problem	Rule out batteries and power first	Pro job	$60–$700	CALLSee the answer: A wiring, fuse or control problem
Wrong mode, a schedule or hold running	The settings are wrong	Look at the mode, the set temperature and any Hold	Free	Not needed	FIXSee the answer: The settings are wrong
Air blows all the time, but it isn't warm or cool	The fan is set to On	Look at the Fan setting	Free	Not needed	FIXSee the answer: The fan is set to On
Thermostat clicks, but nothing runs; or the house stays off	The furnace or AC has a problem	Set it 5 degrees past the room and wait 5 minutes	Pro job	$50–$150	CALLSee the answer: The furnace or AC has a problem
Screen works, settings right, no click, nothing runs	The thermostat or its wiring has failed	Set it 5 degrees past the room and wait 5 minutes	Pro job	$85–$600	CALLSee the answer: The thermostat or its wiring has failed
Smart thermostat says "no power" or "C-wire," or keeps going offline	It needs a C-wire	Look for a C-wire message in the thermostat's app or screen	Pro job	$90–$135	CALLSee the answer: It needs a C-wire
Reads warmer or colder than the rest of the house	Something is fooling the thermostat	Look for sun, lamps, vents or drafts near it	Free	Not needed	FIXSee the answer: Something is fooling the thermostat
Old dial or lever thermostat; house doesn't match	An old or dusty thermostat	Look at its age and for dust	Free	Not needed	FIXSee the answer: An old or dusty thermostat

2026 U.S. averages, rounded. Pro prices include parts and labor. Most companies charge $75–$250 to come out, and many put that toward the repair. Nights, weekends and big cities cost more. See all repair prices

Want proof it's the thermostat, not the furnace?

The 5-minute test

This tells you whether the thermostat is asking for heat or cooling.

Set the mode to Heat or Cool.
Set the temperature 5 degrees past the room: above for heat, below for cooling.
Listen at the thermostat, then wait at least 5 minutes. Don't keep changing it.

The furnace or AC starts: the thermostat works. Check your settings and schedule.
It clicks, but nothing starts: the thermostat is asking. The problem is likely in the furnace or AC.
No click, nothing starts: it may be the thermostat or its wiring. Take the questions at the top.

Not sure it's safe to keep trying?

Stop and call a pro if…

You smell gas or rotten eggs. Leave the house first, then call the gas company from outside.
The furnace or AC breaker trips again after one reset.
The thermostat's wires are loose or damaged.
Water keeps backing up in the drain pan.
You smell burning from the furnace or AC.

Never tape or hold in the furnace door switch. Honeywell says not to tape, bypass or disable it.

Got a quote for a new thermostat?

Fix it, or get a new thermostat?

Honeywell says to troubleshoot before you replace a thermostat. Most problems are batteries, power or a setting. When you do need a new one, HomeGuide lists $140–$350 installed, about $229 on average. Ask yourself three things:

How old is it? HomeGuide says thermostats usually last about 10 years.
Which kind? A basic one installed runs about $100–$200. A smart one runs about $185–$600, and may need a C-wire ($90–$135).
Is the thermostat really the problem? If it clicks but nothing runs, a new thermostat won't help. The furnace or AC needs a look.

Example: about $230 to install a new programmable thermostat is typical. A quote for a new furnace because the thermostat screen went blank is worth a second opinion.

Ran into a word you don't know?

Thermostat words, explained

Read these before you start. The drawings show roughly what each part looks like.

Thermostat: The control on your wall that tells the furnace and AC when to run.
Mode: Heat, Cool, Off or Auto. It tells the thermostat which job to do.
Hold: A setting that keeps one temperature and ignores the schedule.
C-wire: A wire that gives the thermostat steady power from the system. Smart thermostats often need one.
Furnace switch: A light switch on or near the furnace, often on a red plate. It turns the furnace's power on and off.
Door safety switch: A button behind the furnace's front panel. If the panel is loose, it cuts the power.
Breaker: A safety switch in your breaker box. The furnace and AC each usually have their own.
Float switch: A small switch by the drain pan. It cuts power if water backs up.
Condensate drain: The small pipe or hose that carries water away from your AC or high-efficiency furnace.
Compressor: The part in the outdoor AC unit that does the cooling. Systems wait a few minutes before restarting it.

Still have questions?

Questions people ask most

Why is my thermostat blank?

A blank thermostat usually has dead batteries or no power. Put in fresh AA or AAA batteries. Then check the furnace or AC breaker, the switch by the furnace, and that the furnace panel is closed tight.

Is a thermostat that's not working an emergency?

A thermostat that's not working is not an emergency in itself, but a gas smell is. Leave the house without touching the thermostat or any switches. Call the gas company from outside. No heat in freezing weather is urgent. Try the battery and power checks at the top, and call a pro today if the heat stays off.

Why won't my heat or AC come on when the thermostat works?

When the thermostat works but the heat or AC won't come on, check the mode and that the setting is past the room temperature. Then wait 5 minutes. Honeywell says a flashing "Heat On" or "Cool On" is a built-in delay that protects the AC.

Why does my thermostat show one temperature, but the house feels different?

A thermostat can read a different temperature than the house feels when sun, a lamp, a vent or a draft near it fools it. So can a spot in a hallway or on an outside wall. If nothing's near it, the furnace or AC may not keep up.

Do I need a C-wire for a smart thermostat?

Most smart thermostats need a C-wire. HomeGuide says about 90% of thermostats need one to run, and some brands include an adapter. Running a new C-wire costs about $90–$135.

Can I replace a thermostat myself?

You can replace a simple two-wire thermostat yourself with the power off, HomeGuide says. Smart and Wi-Fi models can have 10 to 12 wires, and usually need a pro. A wrong hookup can damage the system.

What do I do with an old mercury thermostat?

Don't throw an old mercury thermostat in the trash. HomeGuide says thermostats from before 2002 may contain mercury. Take it to a hazardous waste drop-off or an HVAC supplier.

Who do I call for a thermostat that's not working?

Call a heating and cooling company (an HVAC tech) if the thermostat still won't work after new batteries and a power check. The same tech can tell if the problem is in the thermostat or in the furnace or AC. Under warranty? Contact the maker first.

How much does a new thermostat cost?

A new thermostat costs about $140–$350 installed, about $229 on average, HomeGuide says. A smart thermostat installed runs about $185–$600.

How we built this

The questions follow the order an HVAC tech checks a thermostat: batteries and power first, then the settings, a 5-minute test, and what's around it. Each answer makes some causes more likely and others less likely.

Prices are 2026 national averages from published repair-cost guides, rounded. Your local prices will differ.

How we make money: some links go to parts stores that pay us a small commission, and some pro listings pay us when you call them. Verdicts come from the diagnosis, not from who pays more.