Wondering what's going on in there?
How your AC starts up, in plain English
Your AC has two halves. One is inside: the furnace or air handler, with a fan that pushes air through your vents. The other is outside: the big metal box with a fan on top. Both need power. And both wait for the thermostat to say "go." When your AC won't turn on, one link in that chain is broken.
- The thermostat says "go." When the room is warmer than your setting, it sends a small signal down thin wires. Then many systems wait 3 to 5 minutes before they start, Carrier says. That protects the main pump outside.
- The breakers send power. The AC usually has its own breaker in the breaker box. The indoor unit often has one too.
- The indoor unit wakes up. Its switch has to be ON and its front panel shut tight. In many homes, a small fuse inside it powers the thermostat. Then its fan blows air over the cold coil and out your vents.
- The drain pan stays dry. The cold coil makes water, the way a cold drink sweats. The water drips into a pan and runs out a pipe. If the pipe clogs, a little float switch shuts the AC off before the pan spills.
- The outdoor unit starts. Power comes through the switch box on the wall. Inside the unit, an on/off switch (the pros call it a contactor) clicks on. A start-up part (a capacitor) gives the motors a push. Then the fan and the main pump (the compressor) run.
So the big question is: which half won't start? If nothing runs at all, look at the thermostat, the power and the drain pan. If air blows inside but the outdoor unit is quiet or humming, look at the outdoor unit. The questions at the top check these in the same order.
Everything runs, but the air is warm? See an AC blowing warm air. Blank screen, or a schedule acting up? See a thermostat that's not working. Water keeps filling the pan? See an AC or furnace leaking water. Heat pump that won't heat in winter? See a heat pump blowing cold air.
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 find out why your AC won't turn on and fix it.
Most companies charge $75–$250 to come out, and many put that toward the repair. If the outdoor unit's main motor is worn out, it can cost $800–$2,800 or more.
Find AC 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
Say this
Hi. My central AC won't turn on. 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.
Answer up to 5 quick questions. We'll give you the words a pro uses.
Answer the questions →More of a list person?
Common reasons an AC won't turn on, 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 |
|---|---|---|---|---|---|
| Thermostat on Off or Heat, or set warmer than the room | A thermostat setting | Set Cool, 5° below the room, fan on Auto. Wait 5 minutes | Free | Not needed | FIXSee the answer: A thermostat setting |
| Screen blank, comes back with new batteries | Dead thermostat batteries | Pop the thermostat off and look for batteries | $0–$10 | Not needed | FIXSee the answer: Dead thermostat batteries |
| "Cool On" blinks, then it starts on its own | It was waiting: the safety delay | Leave it on Cool. Wait 5 full minutes without touching it | Free | Not needed | FIXSee the answer: It was waiting: the safety delay |
| A breaker tripped, or a switch was off | A switch or breaker was off | Reset the AC breaker once. Check the switch box outside and the indoor unit's switch | Free | $75–$250 | FIXSee the answer: A switch or breaker was off |
| Breaker flips off again after one reset | A breaker that trips again | None. Leave the breaker off | Pro job | $150–$2,800+ | CALLSee the answer: A breaker that trips again |
| Front panel on the indoor unit loose or crooked | The indoor unit's front panel was loose | Press the front panel. Is it flat and tight? | Free | Not needed | FIXSee the answer: The indoor unit's front panel was loose |
| Water in the pan under the indoor unit | A clogged drain line shut it off | Look for water in the pan under the indoor unit | $0–$30 | $75–$250 | FIXSee the answer: A clogged drain line shut it off |
| Switches on, panel tight, pan dry, and nothing runs or the screen stays blank | A blown fuse or a wiring short inside | Rule out batteries, switches, the panel and the pan first | Pro job | $75–$700 | CALLSee the answer: A blown fuse or a wiring short inside |
| Smart or new thermostat says "no power" or "C-wire," or keeps going offline | The thermostat's wiring, or it needs a C-wire | Look for a power or C-wire message on the screen or in the app | Pro job | $60–$450 | CALLSee the answer: The thermostat's wiring, or it needs a C-wire |
| Outdoor unit hums, fan doesn't spin | A worn-out start-up part | Hear a hum and see a still fan, from outside the unit | Pro job | $100–$400 | CALLSee the answer: A worn-out start-up part |
| Fan inside runs, outdoor unit silent, switches on, pan dry | A stuck on/off switch | Listen for a "clunk" outside when the AC should start | Pro job | $150–$400 | CALLSee the answer: A stuck on/off switch |
| Outdoor fan spins, but no warm air comes off the top, or it stops after a few seconds | The main pump won't start | Hand over the top: the air isn't warmer than outside | Pro job | $100–$2,800+ | CALLSee the answer: The main pump won't start |
| Outdoor unit runs, but no air from the vents | The indoor fan stopped | Fan set to On: humming at the indoor unit, but no air | Pro job | $100–$2,000 | CALLSee the answer: The indoor fan stopped |
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 trying to start?
The 5-minute look-and-listen test
This is where a tech starts too. It tells you which half won't start.
- Set the thermostat to Cool, 5 degrees below the room. Fan on Auto.
- Don't touch it for 5 full minutes. Changing it restarts the wait.
- Inside: hold your hand at a vent. Is air blowing?
- Outside: stand by the outdoor unit. Don't touch it. Listen for one solid "clunk" when it starts. Then look for the fan spinning.
- Both run: it was just waiting. You're done.
- Air inside, outdoor unit quiet: check the switch box outside and the AC breaker. Then look in the drain pan.
- Outdoor unit hums, fan stays still: set the thermostat to Off and call a pro.
- Nothing runs: check the breakers, the switch by the indoor unit, its front panel and the drain pan.
Not sure it's safe to keep running?
Turn it off right away if…
- The outdoor unit hums or buzzes, but the fan doesn't spin.
- You smell burning or see smoke. Turn off the AC breaker too.
- A breaker flips off a second time. Leave it off. Never tape or hold a breaker on.
- The outdoor fan spins, but the unit shuts off after a few seconds, over and over.
- Water is spilling over the pan by the indoor unit.
To turn it off, set the thermostat to Off. Carrier says to shut the system off and call for service if you hear buzzing or smell burning. Running it like this is how a $200 repair turns into a $2,000 one.
Breakers tripping on other things in the house too? See a breaker that keeps tripping.
Got a big repair quote?
Should you repair it or replace it?
Most no-start repairs are small. A worn start-up part costs about $100–$400, and a stuck on/off switch about $150–$400 (Fixr). The real decision comes with the main pump outside, the compressor. A new one runs $800–$2,800 or more. Ask yourself three things:
- How old is it? The sticker on the side of the outdoor unit shows the date it was made. Most central ACs last 12–17 years.
- What refrigerant does it use? It's on that same sticker. If it says R-22, refills cost a lot, because R-22 isn't made anymore.
- The "$5,000 rule." Multiply the repair price by the AC's age. Over $5,000? Get a price for a new system too. It's a rule of thumb, not a law.
Example: a $1,500 compressor job on a 12-year-old AC is $18,000. Get a replacement quote before you decide.
Ran into a word you don't know?
AC words, explained
Read these before you start. The drawings show roughly what each part looks like.
- Thermostat
- The control on your wall where you set the temperature. It tells the AC when to start.
- Safety delay
- A built-in wait of about 3 to 5 minutes before the AC starts. It protects the main pump outside. Pros call it a short-cycle delay.
- Indoor unit
- The furnace or air handler, in a closet, attic, basement or garage. Its fan pushes air through your vents.
- Outdoor unit
- The big metal box outside with a fan on top. Pros call it the condenser.
- Vent (register)
- The grille in a floor, wall or ceiling where air comes into a room.
- Breaker
- A safety switch in your breaker box. It shuts off power if something draws too much. The AC usually has its own.
- Switch box (disconnect)
- The small gray box on the wall by the outdoor unit. Its handle or switch cuts power to the outdoor unit.
- Furnace switch
- The switch by the indoor unit. It looks like a light switch, often on a red plate. It turns the indoor unit's power on and off.
- Door switch
- A small button behind the indoor unit's front panel. If the panel is loose, it cuts the power.
- Float switch
- A small safety switch on the drain pan or drain pipe. It shuts the AC off when water backs up.
- Drain access cap
- A capped T on the drain pipe near the indoor unit. You pour vinegar in here.
- Drain line
- A skinny white plastic pipe that carries water from the indoor unit to outside.
- Wet/dry vac
- A shop vacuum that can suck up water. Held on the drain pipe's end, it pulls a clog out.
- Air filter
- A flat screen that catches dust before air reaches the indoor unit. It sits behind a big grille or in a slot on the unit.
- C-wire
- A wire that gives the thermostat steady power from the indoor unit. Smart thermostats often need one.
- Low-voltage fuse
- A small fuse on the indoor unit's control board, like a car fuse. It protects the thermostat wires.
- Control board
- The circuit board inside the indoor unit. It runs the fan and passes the thermostat's signal on.
- Start-up part (capacitor)
- A small metal can in the outdoor unit. It gives the motors a push to start, like a jump start.
- On/off switch (contactor)
- The switch inside the outdoor unit that the thermostat turns on and off. It makes a clunk when it closes.
- Main pump (compressor)
- The pump inside the outdoor unit. It's the heart of the AC.
- Indoor fan (blower)
- The fan inside the indoor unit that pushes air through your vents.
Still have questions?
Questions people ask most
Why is my AC not turning on?
An AC that won't turn on may be set wrong, waiting out a short safety delay, or missing power from a switch or breaker. Check the thermostat first. Then wait 5 minutes. Then check the breakers, the switch box outside and the switch by the indoor unit. If the outdoor unit hums but its fan won't spin, turn it off and call a pro (Carrier).
Is an AC that won't turn on an emergency?
An AC that won't turn on usually isn't an emergency, but a burning smell, a loud hum with no fan, or a breaker that trips twice is. Then turn the AC off at the thermostat and call a pro today. Without those signs, it can wait for the free checks. In a heat wave, it's urgent if someone at home is a baby, elderly or sick. Move them somewhere cool.
Why is my outside AC unit not turning on, but the inside fan runs?
When the fan inside runs but the outdoor unit doesn't, start with the AC breaker and the switch box next to the outdoor unit. Then look for water in the drain pan. If it's all fine and the outdoor unit hums, it's probably a worn start-up part. If it's silent, it may be the on/off switch inside it. Both are jobs for a pro.
Why does my thermostat say "Cool On" but the AC isn't running?
A blinking "Cool On" usually means the system is in a built-in delay that lasts about 3 to 5 minutes. It protects the main pump outside (Carrier). Don't keep changing the setting, because that restarts the wait. Still not running after 10 minutes? Check the breakers and the switch box outside.
Why won't my AC turn on after a power outage?
After a power outage, many ACs wait a few minutes before they start, and some thermostats go back to Off. Give it 5 minutes on Cool. Then check that the thermostat still says Cool, and look for a tripped AC breaker (Carrier). Reset a breaker only once.
Why won't my AC turn on after I changed the thermostat batteries or the filter?
After new batteries, many thermostats wait a few minutes or reset to Off, so check the mode and give it 5 minutes. After a filter change at the indoor unit, the front panel may not be pushed in all the way. A loose panel lets a safety switch pop out and cut the power (Honeywell).
Can a clogged drain line stop my AC from turning on?
Yes, a clogged drain line can stop your AC from turning on. Water backs up into the pan, and a small float switch shuts the AC off so nothing floods. Clearing the line with a wet/dry vac usually gets it going again (Carrier).
What should I do before turning on my AC for the first time in spring?
Before you turn on your AC in spring, put in a fresh filter and clear 2 feet around the outdoor unit. Make sure the AC breaker and the switch box outside are on. Then set the thermostat to Cool, a few degrees below the room, and wait 5 minutes (HVAC.com, U.S. Department of Energy). Pick a warm day to test it. If the breaker trips, don't keep resetting it.
Can I test my AC in the winter?
It's best not to test a central AC when it's colder than about 60 degrees outside. Many HVAC sources say to wait until it's at least 60 degrees out, PickHVAC among them. Below that, the oil in the main pump is thick, and starting it can hurt it. Some newer ACs won't start at all in the cold. That's on purpose, not a problem.
I have a heat pump. Is this the right guide?
This guide works for a heat pump that won't cool in summer, because the switches and checks are the same. If your heat pump won't heat, or blows cold air in winter, use our heat pump guide instead. Its outdoor unit defrosts itself, and that can look like a problem when it isn't (Lennox).
How much does it cost to fix an AC that won't turn on?
Fixing an AC that won't turn on can cost nothing, if it's a setting, a switch or the drain line. A worn start-up part costs about $100–$400, a stuck on/off switch $150–$400, and a new main pump $800–$2,800 or more (Fixr). Most companies charge $75–$250 to come out, and many put that toward the repair.
Who do I call when my AC won't turn on?
Call a heating and cooling company (an HVAC tech) when your AC won't turn on after the free checks. Try the thermostat, the 5-minute wait, the breakers, the switch box outside and the drain pan first. Call an electrician only if the problem is in your home's wiring, not the AC. Renters should call the landlord first. If your AC is under 10 years old, the company that put it in may cover parts under warranty.
How we built this
The questions follow the order an HVAC tech checks an AC that won't start: the thermostat, the 5-minute wait, which half runs, the power switches, the drain pan, then the parts inside each unit. 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.
- Carrier, AC unit not turning on: reasons why and what to do (read October 2026)
- Carrier, "Cool On" flashing on thermostat: what it means and how to fix it (read October 2026)
- Carrier, troubleshoot an AC not working (read October 2026)
- Carrier, how to clean an AC drain line (read October 2026)
- Carrier, why is my AC leaking water? (read October 2026)
- Honeywell Home support, how do I fix a blank display on my RTH6360 thermostat (September 2025)
- Honeywell thermostat not working: troubleshooting guide, HoneywellStore.com, run under license from Honeywell (read October 2026)
- HVAC.com, why is my thermostat's screen blank (July 2025)
- HVAC.com, what to do before turning your AC on after winter (February 2025)
- U.S. Department of Energy, maintaining your air conditioner (read October 2026)
- Lennox, heat pump blowing cold air (February 2026)
- Lennox, understanding the heat pump defrost cycle (November 2025)
- PickHVAC, what is the coldest temperature to run an AC (June 2025)
- Fixr, AC repair cost (July 2026)
- HomeGuide, AC repair cost (August 2025)
- HomeAdvisor, AC repair cost (June 2026)
- HomeGuide, thermostat installation cost (August 2025)
- HomeServe, thermostat replacement cost (January 2024)
- This Old House, AC capacitor cost (April 2026)
- Angi, AC compressor cost (July 2026)
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.