Troubleshooting · Won't run

AC Not Working After a Power Outage? What to Check and How to Prevent It

Power is back on, but the AC won't start or doesn't cool.

By Mark Curant·Updated October 14, 2026

Most likely causes

A built-in restart delay: it hasn't started yet
The thermostat lost its settings or schedule
A breaker tripped when power came back
A blown low-voltage fuse or transformer from a surge
A failed capacitor or control board from a surge or brownout

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)

Wait 10 minutes. Many systems hold off restarting to protect the compressor.
Thermostat: check the mode, temperature and clock, and replace the batteries.
Breakers: check both the indoor and outdoor unit breakers.
Smell: any burning smell at the indoor unit, outdoor unit or panel means leave it off and call.

Causes, signs and repair costs

Restart delay

DIY

Indoor fan may run; outdoor unit starts after a few minutes

Typical cost: $0

Thermostat reset or lost settings

DIY

Blank or flashing screen, wrong mode or clock

Typical cost: $0–$10

Tripped breaker

DIY or pro

Breaker in the middle position

Typical cost: $0 to reset; service call if it trips again

Blown low-voltage fuse or transformer

Pro

Thermostat has no power even with new batteries

Typical cost: $75–$200 service call plus parts

Failed capacitor

Pro

Outdoor unit hums but doesn't start

Typical cost: $150–$400

See: Outdoor unit not running →

Damaged control board

Pro

Blinking error lights on the board, no response to the thermostat; common after lightning or surges

Typical cost: $75–$200 service call plus the board

Cause	Signs	Who fixes it	Typical cost
Restart delay	Indoor fan may run; outdoor unit starts after a few minutes	DIY	$0
Thermostat reset or lost settings	Blank or flashing screen, wrong mode or clock	DIY	$0–$10
Tripped breaker	Breaker in the middle position	DIY or pro	$0 to reset; service call if it trips again
Blown low-voltage fuse or transformer	Thermostat has no power even with new batteries	Pro	$75–$200 service call plus parts
Failed capacitorSee: Outdoor unit not running →	Outdoor unit hums but doesn't start	Pro	$150–$400
Damaged control board	Blinking error lights on the board, no response to the thermostat; common after lightning or surges	Pro	$75–$200 service call plus the board

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
Give it 10 minutes
Leave the thermostat calling for cooling and wait. Restart delays of 3–5 minutes are common, and some units wait longer after a power cut.
2
Reset the thermostat
Check the mode, set temperature and clock, and replace the batteries. Smart thermostats may need to reconnect to Wi-Fi before a schedule works again.
3
Power-cycle the system
Turn the thermostat off, switch off both breakers for a minute, then turn them back on and set the thermostat to Cool. This clears many control-board lockouts.
4
Reset a mini-split
For a ductless system, turn it off at the breaker for a minute, then restart it with the remote. If the indoor unit shows a blinking light or error code, note the code and pattern; it tells the technician what failed.
5
Stop if anything is wrong
If a breaker trips again, you smell burning, or the outdoor unit hums without starting, switch everything off and call. Repeated restarts on a damaged part can turn a small repair into a compressor replacement.

Call a technician if

A breaker trips again after one reset
There's a burning smell, or scorch marks on wiring
The thermostat has no power with fresh batteries
The outdoor unit hums but won't start

Red flags on a quote

A new system recommended for surge damage before the capacitor, fuse and board are tested
Ask whether your homeowner's insurance covers surge or lightning damage before paying for a large repair

Facing an expensive repair on an older system? Run it through the repair vs replace calculator, or read the $5,000 rule.

How to protect your AC next time

Where I run my stores, outages and brownouts are part of normal life, and I've lost refrigeration compressors to brownouts. Those units now run on protectors that cut power when the voltage drops and wait before reconnecting. For the air conditioners, we stagger the restarts so the units don't all start at once when power comes back, and before a hurricane every unit and its breaker gets switched off.

Most of that is free: turn the AC off when the power goes out, and wait a few minutes after it's stable before turning it back on. Beyond that, a voltage monitor at the outdoor unit and a whole-house surge protector are the main options.

See How to protect your AC from power surges, brownouts and outages.

Frequently asked questions

Why won't my AC turn back on after a power outage?

Often it just hasn't restarted yet: many systems wait several minutes after power returns. If it still won't run after 10 minutes, check the thermostat settings and batteries, then the breakers. A surge when power comes back can also blow the low-voltage fuse or damage a capacitor or control board.

How do I reset my AC after a power outage?

Turn the thermostat off, switch off the breakers for the indoor and outdoor units for about a minute, turn them back on, then set the thermostat to Cool and wait 5–10 minutes.

Should I turn off my AC during a power outage?

Yes. Turning the system off at the thermostat (and the breakers, if a storm is coming) means it won't try to start during the surges and brownouts that often come when power is restored. Wait a few minutes after power is stable before turning it back on.