AC Won't Turn On? What to Check Before You Call
Nothing happens when you lower the thermostat: no fan, no outdoor unit.
By Mark Curant·Updated
Most likely causes
- Thermostat: dead batteries, wrong mode or a schedule overriding it
- A tripped breaker or blown fuse
- The service switch by the indoor unit is off, or the panel door isn't closed
- A full drain pan has tripped the float (safety) switch
- A blown low-voltage fuse or failed transformer
Before you open anything
Turn the system off at the thermostat, then switch off power at the breaker and at the disconnect beside the outdoor unit. Never touch the capacitor, even with the power off: it can hold a dangerous charge. Refrigerant work is for licensed technicians only. Reset a tripped breaker once; if it trips again, leave it off.
Check these first (free, 5 minutes)
- Thermostat screen: blank means batteries or power. Set it to Cool, 3–5°F below room temperature, and switch off any schedule or "hold" setting.
- Wait 5–10 minutes. Many systems have a restart delay after a setting change or power interruption.
- Breakers: there are usually two, one for the indoor unit (furnace or air handler) and one for the outdoor unit.
- Service switch: a light-style switch on or near the indoor unit. It's easy to flip off by mistake.
Causes, signs and repair costs
Thermostat batteries, settings or schedule
DIYBlank screen, "Heat" or "Off" mode, a schedule holding a higher temperature
Typical cost: $0–$10
Failed thermostat
DIY or proScreen on and set correctly, but nothing starts; works with a test jumper (pro)
Typical cost: $100–$400
Tripped breaker or blown fuse
DIY or proBreaker in the middle or off position; fuse in the outdoor disconnect blown
Typical cost: $0 to reset; service call if it trips again
Service switch off or panel door open
DIYSwitch by the indoor unit off; the blower door has a safety switch that cuts power when it isn't seated
Typical cost: $0
Full drain pan (float switch)
DIY or proWater in the drain pan or a secondary pan; the system shuts off to prevent a leak
Typical cost: $10–$20 DIY, $75–$200 pro
See: AC leaking water →Blown low-voltage fuse or failed transformer
ProThermostat dead even with fresh batteries; often after a surge or a wiring short
Typical cost: $75–$200 service call plus parts
Failed capacitor or contactor
ProIndoor fan runs but the outdoor unit doesn't
Typical cost: $150–$400 capacitor, $150–$300 contactor
See: Outdoor unit not running →| Cause | Signs | Who fixes it | Typical cost |
|---|---|---|---|
| Thermostat batteries, settings or schedule | Blank screen, "Heat" or "Off" mode, a schedule holding a higher temperature | DIY | $0–$10 |
| Failed thermostat | Screen on and set correctly, but nothing starts; works with a test jumper (pro) | DIY or pro | $100–$400 |
| Tripped breaker or blown fuse | Breaker in the middle or off position; fuse in the outdoor disconnect blown | DIY or pro | $0 to reset; service call if it trips again |
| Service switch off or panel door open | Switch by the indoor unit off; the blower door has a safety switch that cuts power when it isn't seated | DIY | $0 |
| Full drain pan (float switch)See: AC leaking water → | Water in the drain pan or a secondary pan; the system shuts off to prevent a leak | DIY or pro | $10–$20 DIY, $75–$200 pro |
| Blown low-voltage fuse or failed transformer | Thermostat dead even with fresh batteries; often after a surge or a wiring short | Pro | $75–$200 service call plus parts |
| Failed capacitor or contactorSee: Outdoor unit not running → | Indoor fan runs but the outdoor unit doesn't | Pro | $150–$400 capacitor, $150–$300 contactor |
Typical U.S. installed prices in 2026. Most repairs also carry a service call fee of $75–$200, often credited toward the repair. See AC repair costs in 2026 for the full list.
Step by step
- 1
Check the thermostat
Replace the batteries if it has them, even if the screen is on. Set Cool, fan Auto, and a temperature well below the room. Turn off any schedule or vacation hold. Smart thermostats can lose their connection to the system; check the app for an error.
- 2
Wait for the restart delay
Give it 5–10 minutes. Many thermostats and outdoor units wait before starting to protect the compressor, and it's easy to mistake that for a fault.
- 3
Reset breakers once
Find the breakers for the indoor and outdoor units. A tripped breaker sits in the middle; push it fully off, then on. Reset only once. If it trips again, leave it off and call a technician.
- 4
Check the switches
Make sure the service switch near the indoor unit is on, and that the blower compartment door is fully seated. Outside, check that the disconnect box beside the outdoor unit is switched on or its pull-out block is fully inserted.
- 5
Look in the drain pan
If there's standing water in the pan under the indoor unit, or in the secondary pan beneath it, the float switch has probably shut the system off. Clearing the drain line usually brings it back. See the leaking water page for how.
Call a technician if
- A breaker trips again after one reset
- You smell burning, or see scorch marks in the disconnect box or panel
- The thermostat has power and the right settings, but the system doesn't respond
- You hear clicking or humming, but nothing starts
Red flags on a quote
- A new thermostat recommended without checking the fuse and transformer
- A diagnostic fee above $200 outside a major city
- Pressure to replace the system for a no-start problem before the basics are tested
Facing an expensive repair on an older system? Run it through the repair vs replace calculator, or read the $5,000 rule.
Frequently asked questions
Why won't my AC turn on even though the thermostat is on?
The most common reasons are a tripped breaker, the service switch near the indoor unit being off, the blower door not being closed, or a full drain pan tripping the float switch. If all of those check out, the low-voltage fuse or transformer may have failed, which needs a technician.
Is there a reset button on an air conditioner?
Most central systems don't have a single reset button. The equivalent is turning the system off at the thermostat, switching off both breakers for a minute, then turning them back on and waiting 5–10 minutes. Some units have a reset on the control board or a high-pressure switch, but those should be left to a technician, because a tripped safety switch means something caused it.
Can a clogged drain stop the AC from turning on?
Yes. Many systems have a float switch in the drain pan or drain line that shuts the AC off when water backs up, to stop it flooding the house. Clearing the drain line usually lets it restart.
Related problems
AC leaking water
Water around the indoor unit, a dripping ceiling or a full drain pan.
Outdoor unit not running
The indoor fan blows but the outside unit is silent, humming or its fan is still.
AC not working after a power outage
Power is back on, but the AC won't start or doesn't cool.
Fixed it? Check what it costs to run
If your system is old enough to keep breaking, compare its running cost with a new one before the next repair.
Open the SEER savings calculator