An air conditioner that won’t turn on at all is almost never “just dead”. Its power supply, the thermostat’s low-voltage signal or a safety switch that shuts the system down on purpose has stopped it. A tripped breaker, dead thermostat batteries and a full condensate pan are things you can find yourself. A failed capacitor, a burned contactor or a blown fuse in the outdoor disconnect needs a technician with a meter.
First, find out which part is dead
Check three spots and note what you see and hear:
- The thermostat. Is the screen lit? Is it on Cool, with the setting below the room temperature?
- The indoor unit (the furnace or air handler in a closet, the garage or the attic). When you lower the setting, does its blower start?
- The outdoor unit. Is it silent, humming, or clicking?
Those answers split the problem four ways:
- Blank thermostat and nothing anywhere. Many thermostats take their power from a small transformer inside the indoor unit. A screen that stays dark with fresh batteries usually means that unit has no power: a breaker, its own switch, a door safety switch or a blown low-voltage fuse on the control board.
- Blower inside works, nothing outside. The call for cooling reaches the house side, but the outdoor unit never starts. Look at its breaker and disconnect; after that it’s a fuse, the contactor or the wiring between them.
- Outdoor unit hums or buzzes, nothing spins. Power is there, but a motor can’t get going. A weak capacitor is the most common reason. Switch the system off; letting it hum strains the compressor.
- Breaker trips the moment it starts. Something is pulling far too much current: a shorted fan motor, a failing compressor or damaged wiring.
What each symptom usually means
| Symptom | Likely cause | What it takes |
|---|---|---|
| Thermostat screen blank | Dead batteries, no power at the indoor unit, failed thermostat | Batteries; power check; thermostat test |
| Screen on, no click, nothing starts | Wrong mode or setting, loose wire, blown board fuse | Settings; wiring and fuse check |
| Air handler blows, condenser never starts | Outdoor breaker or disconnect off, blown fuse, failed contactor | Fuse or contactor replacement |
| Condenser hums, its fan stands still | Failed capacitor, sometimes a seized fan motor | Capacitor or motor replacement |
| Nothing runs, water in the pan under the indoor unit | Condensate float switch cut the system off | Clearing the drain, resetting the switch |
| Breaker snaps off at start-up | Short in a motor, the compressor or a wire | Testing before anyone restarts it |
| Won’t start right after a power outage | Built-in delay protecting the compressor | Wait a few minutes; call if it never starts |
Checks that are safe for you
- Thermostat: Cool mode, a setting several degrees under the room temperature, new batteries if it uses them.
- Breakers: find the ones for the outdoor unit and for the furnace or air handler. A tripped breaker often rests in the middle position; push it fully off, then back on. Do this one time only.
- Indoor power switch: most furnaces and air handlers have one mounted nearby, and it gets bumped off more often than you’d think.
- Outdoor disconnect: the box on the wall beside the outdoor unit. Make sure its pull-out block or lever is in the on position. Don’t open it further or touch the fuses inside.
- Condensate pan: look for standing water in the pan under an attic or closet unit. Water there means a clogged drain, and the float switch is doing its job by stopping a ceiling leak.
- Wait out the delay: after an outage or a breaker reset, many thermostats and outdoor units won’t let the compressor restart for several minutes.
Jobs for a technician
- An outdoor unit that hums, clicks or buzzes without starting, or a breaker that trips a second time.
- A burning smell, scorched wiring or melted insulation at either unit. If you smell gas near a gas furnace, leave the house and call from outside.
- The capacitor, the contactor and the fuses in the disconnect. A capacitor can hold a dangerous charge with the power off, and the disconnect carries full line voltage. These are not owner jobs.
- A system still under a manufacturer’s warranty, where parts work done by the owner can complicate a claim.
The technician traces voltage from the breaker to the compressor, tests the capacitor against its label rating, checks the contactor for pitting and measures motor current before replacing anything; see AC repair.
Cost and repair time
Most AC repairs land between $89 and $500, and bigger parts run higher. A capacitor, contactor or fuse is near the bottom of that range and is usually replaced on the first visit. A fan motor or a compressor fault costs more. The diagnostic fee is waived if you go ahead, and nothing is done until you approve the price. A thermostat that has failed outright is replaced for $150 to $400; see thermostat installation and our page on a thermostat that isn’t working.
If the system does start but shuts off again after a few minutes, see AC short cycling.
A San Jose note
The first hot afternoon of the year often arrives after the AC has sat unused since fall. That’s when a capacitor or contactor weakened by last summer’s heat finally refuses to start a motor. Run the system for a test on a mild spring day, before you need it, and you have time to book a repair or an AC tune-up instead of waiting out a hot spell.