What to do when your thermostat is set to cool but the AC isn't turning on

Find out why your thermostat is set to cool your home but the air conditioner isn't turning on, from issues with the thermostat itself to the AC unit.

(Image credit: Getty Images)

Turning on the AC on a hot day and finding out it's not working is never ideal. By that time, it's too late, and you're already in need of relief and comfort from the heat.

If you've already checked why your air conditioner isn't blowing cold air, the problem could be coming from somewhere else. AC units are complex machines, so the issue could come from various places.

It's also worth cleaning your air conditioner, as this will reveal whether anything is too clogged to work properly. But if all else fails, we spoke to the AC experts to find out what could be causing the problem. Read on for what to do.

What to do when a thermostat is set to cool but the AC isn't turning on

Frustratingly, the problem can be a number of issues. Brad Gall, owner of BG Electrical & Air Con, explains that the problem 'can lie anywhere, from your circuit breaker to the indoor or outdoor units.'

1. Check the power

Start off with the potential causes that would be easy to fix. 'It could be something as simple as the thermostat is out of batteries and you can't adjust the settings because it's dead,' explains Melanie Powers, president of Goodberlet Home Services. If the display is off, this could be the culprit, then all it takes is finding the right batteries to replace with.

Failing this, Brad Gall explains that 'one of the most common reasons is a tripped circuit breaker, and it's among the easiest to diagnose. If it's off for the AC, just flip it back on. But if it happens again, you'll have no other safe resort but to call an electrician.'

Air conditioner unit outdoors, next to large plant vase

(Image credit: Getty Images)

2. Reset the system

Brad Roberson, President of Aire Serv, adds that the HVAC system may simply need to be reset: 'If the HVAC unit is not turning on and the room temperature is higher than the thermostat setting, the unit may need to be reset.'

Some homeowners may be familiar with how to do this, and hopefully, this can allow the unit to come back online. 'If resetting the breaker doesn’t work, it is best to consult an HVAC service professional to diagnose and fix the problem.'

3. Test the other functions

If resetting the system doesn't help, you can narrow it down by seeing what else works.

'Set [the HVAC] to heat or dry or fan only - literally any other function. If everything else works, there could be any number of issues,' Brad Gall suggests.

It could be related to an ice build-up, a clogged filter, or even your air conditioner leaking water.

4. Check air filters & drain line

A small orange smart thermostat on a beige wall above a brown sood side boared with red and yellow flowers

(Image credit: Nest)

'If you haven't cleaned or replaced the air filters recently, even they could have caused it if they are dirty enough,' Brad Gall says. Luckily, this is as simple as cleaning or replacing the filters, although it's unlikely that this is the cause.

It could also be the drain line that's causing the problem. 'Take a look at the PVC pipe that's sticking out of the indoor unit and going outside. It's a drain line and you should check it for leaks from time to time.'

'Also, look around the indoor unit itself for moisture. A leaky or clogged line will cause the drain pan to flood, which will then trigger an automatic shutdown of the system.'

5. Use caution

If the above fixes haven't solved the problem, chances are there's an internal component at fault. Brad Robertson emphasises:

'There are a lot of sensitive components inside the HVAC unit and outside condenser unit. Homeowners should avoid opening the HVAC unit to work on these components. If resetting the HVAC unit does not work to fix the issue, homeowners should consult an HVAC service professional.'

To avoid future issues, be sure to memorize how often a HVAC system should be serviced.

And while you're waiting for it to be fixed, browse the best fans and find out more about the pros and cons of fans vs air conditioners, to potentially find a way to avoid stress down the line.

Dan Fauzi
Home Tech Editor

Dan is the Home Tech Editor for Homes & Gardens, covering all things cleaning, smart home, sound and automation across the Solved section. Having worked for Future PLC since July 2023, Dan was previously the Features Editor for Top Ten Reviews and looked after the wide variety of home and outdoor content across the site, but their writing about homes, gardens, tech and products started back in 2021 on brands like BBC Science Focus, YourHomeStyle, Homes & Antiques and Gardens Illustrated.

Dan is based in Bristol, UK with a BA in Philosophy and an MA in Magazine Journalism. Outside of work, you'll find them at gigs and art galleries, cycling somewhere scenic, or cooking up something good in the kitchen.