Symptom guide
AC not turning on: the four checks, in order
By the HeatAndAirCost editors · Last updated October 5, 2026 · How we research prices
What people commonly report
Nothing happens when the thermostat calls for cooling. No indoor fan, no outdoor hum, sometimes a blank thermostat screen. Sometimes the system ran earlier in the day and then quit. This page is about the air conditioner. For a furnace that will not start, see furnace not turning on.
What manufacturers say the causes can include
Bryant’s troubleshooting runs in a fixed order: thermostat, then power, then airflow, then condensate drainage. Its list of the most common culprits is “an unresponsive thermostat, a tripped circuit breaker, a clogged air filter, or a triggered condensate float switch”[1].
- Thermostat. A blank display means no power reaches the thermostat, per Trane[2]. DOE’s list for a unit that will not turn on starts with making sure the thermostat works: “it might need new batteries or might need to be replaced entirely”[5].
- Power. A tripped breaker or blown fuse stops the system, per Lennox[3].
- Condensate float switch. When the drain backs up, the overflow switch shuts the compressor, outdoor fan and indoor fan down. Trane notes that a sudden shutdown on a hot day can mean a clogged condensate line or pan[4].
- Capacitor or contactor. Lennox says either can stop the outdoor unit from starting even with a call for cooling[3].
- Worn controls. DOE says compressor and fan controls can wear out from the system turning off and on too often, and to have a professional check the electrical connections[5].
How to find out
- Thermostat: set to COOL, a few degrees below room temperature. Lennox suggests 3 to 5 degrees[3]. Replace batteries if it has them.
- Power: find the breakers for the indoor unit and the outdoor unit. Check the furnace or air handler power switch, which Carrier notes may look like a standard light switch[7].
- Airflow: pull the filter. A packed filter restricts airflow, per Bryant’s order of checks[1].
- Drain: if you can see the drain pan under the indoor unit, look for standing water. Bryant pairs “AC will not start and water is in the drain pan” with a clogged condensate drain or an activated float switch[1]. Our water leaking from AC page covers the drain.
Caution
The breaker advice is not the same everywhere
Lennox says a homeowner can reset a tripped AC breaker once, and that an immediate re-trip needs a professional[3]. Trane says a tripped HVAC breaker means a system problem caused it, and to have that fixed before turning the system back on[2]. Bryant adds that repeated resets “can damage the system and create a fire hazard”[6]. None of them supports resetting it more than once.
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Try this while you wait
The thermostat and filter checks are the two a homeowner can finish alone. Match the battery type to your thermostat and the filter to the size printed on the old one.
As an Amazon Associate we earn from qualifying purchases. It does not change your price. These are screening or upkeep items. None of them is a repair, and none replaces a licensed technician.
When to stop and call
- The breaker trips again.
- The drain pan is full and the system will not restart after it is cleared.
- The outdoor unit hums or clicks but never starts.
- You smell burning. See burning smell for the shut-off step.
Related guides
What the repair can cost
Related problems
Sources
This page is general information collected from the official and manufacturer sources listed above. It is not an inspection or a diagnosis of your system, and only a qualified technician who sees the equipment can tell you what is wrong. HeatAndAirCost is not affiliated with the U.S. Department of Energy, Bryant, Trane, Lennox, Carrier. Found an error? hello@heatandaircost.com.
Getting local help
Finding a qualified technician
We do not inspect systems, rate contractors, or recommend any particular company. Whoever you call, ask for the license number and look it up on your state or county licensing board’s public site before work starts. Refrigerant work also requires an EPA Section 608 certified technician. Our contractor hiring checklist covers licenses, insurance, written quotes and red flags.
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People also ask
AC not turning on: FAQs
Why won't my AC turn on?
Bryant names the most common culprits as an unresponsive thermostat, a tripped circuit breaker, a clogged air filter, or a triggered condensate float switch, and checks them in that order: thermostat, power, airflow, condensate drainage. A failed capacitor or contactor can also keep the outdoor unit from starting, per Lennox.
Should I reset the breaker if my AC won't turn on?
Manufacturers differ. Lennox says you can reset it once if you are comfortable doing so, and that an immediate re-trip means an electrical hazard needing a professional. Trane says a tripped HVAC breaker means a system problem caused it, and to have it fixed before turning the system back on. Bryant warns that repeated resets can damage the system and create a fire hazard. All three agree: no repeated resets.
Why did my AC shut off and not come back on?
One common reason is the condensate overflow switch. Trane explains that when water backs up in the drain pan or line, the switch shuts down the compressor, outdoor fan and indoor fan, so an AC that stops in the heat of the day may have a clogged condensate line or pan.
The indoor fan runs but the outside unit doesn't. Is that 'not turning on'?
That is a different symptom: air moves, but it is not cooled. Lennox covers it as an outdoor unit that will not start, and our AC blowing warm air page walks through it.