Symptom
AC will not turn on at all
Nothing runs. No blower, no outdoor unit, or a thermostat that has gone blank. There are a handful of causes and two of them you can check safely in five minutes.
Where to start
The first question is whether anything runs. If the indoor blower runs but the outdoor unit does not, that is a different problem from total silence, and it points at the outdoor circuit or the contactor. If nothing runs anywhere and the thermostat is blank, the indoor unit has lost power.
Two of the most common causes are things you can check yourself without opening anything: a tripped breaker, and the service switch beside the indoor unit that looks like a light switch and gets turned off by mistake.
Safe checks, in order
- Thermostat display. Blank, or showing a call for cooling?
- If it takes batteries, try fresh ones
- Breaker panel. Look for a tripped breaker, including one sitting mid-way rather than fully off
- The switch on or near the indoor air handler. It looks like a light switch and is frequently the answer
- The disconnect box beside the outdoor unit, in case it has been pulled
- Standing water in or under the drain pan, which suggests a tripped float switch
Reset a tripped breaker once. If it trips again, leave it off and call.
Call (866) 427-9264Causes
What is normally responsible
A tripped float switch
This deserves first place because it is so often overlooked and because it is so common in this climate. When the condensate drain blocks, water rises in the drain pan and a float switch cuts the control circuit to stop the system before the overflow reaches a ceiling. The system goes completely dead with no warning and no noise, usually on a hot afternoon when the coil is producing the most water.
Resetting it without clearing the drain simply repeats the shutdown, and bypassing it is how a service call turns into a ceiling repair. The fix is clearing the line. See clogged drain lines.
Power problems
A tripped breaker, a pulled disconnect, or the indoor service switch in the off position. Between them these account for a large share of dead systems. The service switch in particular catches people out, because it is often in an attic or a closet and looks exactly like a light switch somebody flipped while looking for the light.
A failed contactor
The contactor switches power to the outdoor unit. Its contacts erode over years of switching under load, and eventually they stop making a connection. The result is an indoor blower that runs normally while the outdoor unit does nothing. Contactors can also stick closed, which produces an outdoor unit that never shuts off.
A failed capacitor
A capacitor that has failed completely leaves motors unable to start. You may hear a hum from the outdoor unit, or nothing. This is a routine repair and one of the more likely causes in a system with a lot of accumulated runtime. See capacitor replacement.
Transformer or control board failure
The low-voltage control circuit runs the thermostat and the relays. A failed transformer or a fault on the control board takes the whole system down and often blanks the thermostat with it. Less common than the causes above, and it needs proper testing rather than parts swapping.
Thermostat faults and wiring
Genuine thermostat failures happen but are less common than they get blamed for. Loose or corroded low-voltage wiring, particularly at the outdoor unit where it is exposed to weather, causes intermittent no-start conditions that are easy to misread as a thermostat problem.
Reading the pattern
What runs and what does not
- Nothing anywhere, thermostat blank. Indoor power. Breaker, service switch, transformer, or a tripped float switch.
- Thermostat on, blower runs, outdoor unit silent. Outdoor circuit or contactor. Check the outdoor breaker and disconnect first.
- Outdoor unit hums but nothing turns. Power is arriving. A capacitor is the leading suspect.
- Everything starts, then trips the breaker within seconds. A genuine electrical fault. Leave the breaker off and call.
- Stops and starts by itself repeatedly. That is a different pattern. See short cycling.
Leave this part to a technician
Do not open the electrical panel on either unit, test or replace a capacitor, or bypass a float switch to get cooling back.
A bypassed float switch will run happily right up until the point the drain pan overflows into the ceiling below. It is a safety device that has been fitted precisely because that outcome is expensive.
A breaker that trips repeatedly is not a faulty breaker. It is a breaker doing its job.
Related pages
Float switch keeps tripping
The safety device that shuts a healthy system down deliberately.
Clogged AC drain line
What causes the float switch to trip in the first place.
Capacitor replacement
Humming outdoor unit that will not start.
AC blowing warm air
When the indoor side runs but the outdoor side does not.
Emergency AC repair
If the house is heating up and this cannot wait until morning.
AC repair
What a full diagnostic visit covers.
Questions about a system that will not start
My thermostat screen is blank. Is the thermostat broken?
What is a float switch and why would it stop my AC?
The breaker tripped. Can I just reset it?
It worked this morning and now nothing. What changed?
If the breaker has tripped more than once, that is a call rather than another reset.
Call (866) 427-9264Checked the obvious and still nothing?
Beyond breakers, switches and float switches, this needs test equipment rather than more guesses. 24/7 emergency HVAC service.