AC Won't Turn On at All? Run These 5 Free Checks
No hum, no fan, no air — the system is just dead. Before you call, know that "dead" systems are revived every day by a flipped breaker or a safety switch doing its job. Five checks, about ten minutes, no tools.
Standing rule, every page: if you ever smell gas — rotten eggs — leave the house and call your gas company from outside. That outranks everything below.
1. Thermostat basics
Mode on COOL, target set at least 3 degrees below the room temperature, fan on AUTO. Blank screen? Fresh batteries. Then wait five full minutes — most AC systems have a built-in startup delay to protect the compressor, and impatience gets misread as failure.
2. Two breakers, not one
Central air usually has two breakers: one for the indoor unit (often labeled furnace or air handler — yes, even for cooling) and one for the outdoor unit. Check both. Reset a tripped one exactly once: fully OFF, then ON. If it trips again, that's an electrical fault — stop resetting and call.
Some outdoor units also have a shutoff box mounted on the wall right next to them. Make sure it's on (or the pull-out block is seated) — landscapers and painters pull these and forget.
3. The light switch that isn't
On or near your indoor unit is a switch that looks exactly like a light switch. It cuts power to the whole system, and it gets flipped by mistake constantly. If it's off, flip it on and wait a few minutes.
4. The float switch (the sneaky one)
Your AC makes water — condensation — and drains it away. If that drain clogs, a small safety called a float switch shuts the system down before the pan overflows through your ceiling. The clue: the system died on a humid day, and you may find water in or around the drain pan under the indoor unit.
If you see standing water, the drain line is the suspect. Some homeowners clear the line themselves at the outside drain stub; if that's not something you're set up for, it's a quick, inexpensive pro visit — and nothing else will fix it until the water clears.
5. Any recent work?
New thermostat? Electrical work? Water heater replaced? A system that died right after someone worked nearby is telling you where to look. A thermostat wired wrong will kill a healthy system — if you swapped the thermostat yourself and the AC died with it, that's suspect number one.
Still nothing?
If all five check out, the failure is likely a capacitor, contactor, or control issue — common repairs, but a technician's job.
Run our free troubleshooter first. It asks what a good shop would ask on the phone, records what you've ruled out, and gives you a plain-English summary for the service call.
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