AC not turning on? Here's what to check first
An AC that won't turn on is usually a tripped breaker, a blown fuse, a thermostat problem, or a tripped safety switch — most often a full condensate line shutting the system off on purpose. Nine causes, in the order to check them.
After power returns, the compressor waits up to 3 minutes by design — a unit that seems dead may just be counting down. And if you smell burning at the outdoor unit, leave the power off and call.
On this page
The 10-minute checks
Free, in order. Most no-start calls end here — the outdoor disconnect and the condensate float are the two people miss most.
- Thermostat. Set to COOL, 3°F below room temp, and swap the batteries — a dead cell blanks the whole system.
- Breaker. Reset the AC breaker once. If it trips again, stop — that's an electrical fault, not a reset problem.
- Outdoor disconnect. The gray box on the wall by the outdoor unit. A pulled fuse block here is the most-missed cause.
- Indoor switch. The furnace or air-handler has its own light-switch-style cutoff. Someone may have flipped it off.
- Air filter. A filter clogged solid can ice the coil and lock the system out. Pull it and check.
- Condensate float. Full drain pan or a clogged line? The float switch shuts the AC off on purpose — clear it first.
- 3-minute wait. After power returns, the compressor holds off up to 3 minutes by design. Wait before you panic.
Still dead after all seven? The cause is a capacitor, contactor, or wiring — licensed-technician territory. The groups below name each by its signature.
Checks done, still no cooling?
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Power & electrical causes
Tripped breakerFree–PRO · $0
The compressor's startup draw, or a short, trips the breaker. Reset it once. A breaker that trips again is signalling a real fault — a shorted compressor or burnt wiring — and repeated resets are a fire risk. One re-trip means stop and call.
Blown fuse at the disconnectDIY–PRO · $10–$40
The outdoor disconnect box holds a fuse block that can blow after a surge or power blip. A matched replacement fuse is cheap, but a fuse that blows again points to a downstream short — that's a technician job.
Failed capacitorPRO · $150–$400
The most-routed summer repair. Signature: the outdoor unit hums but won't start, or the fan won't spin. The capacitor is the jolt that starts the motor, and heat kills them. Installed it runs $150–$400 — see the AC repair cost breakdown. Do not open the panel: a capacitor holds a charge even with the power off.
Burnt wiring or contactsPRO · $150–$500
Melted insulation, a burnt smell, or scorched terminals are a fire hazard and a licensed-only repair. If you smell burning, leave the power off and call.
Pitted contactorPRO · $150–$350
The contactor is the relay that switches the outdoor unit on. Its contacts pit and stick over time, so the unit either won't start or won't shut off. A technician swaps it in one visit.
Thermostat & control causes
Settings or dead batteriesFree · $0
A blank screen is usually dead batteries. Confirm COOL mode and a setpoint below room temperature before anything else — most "won't turn on" calls end here.
Miswired or aging thermostatPRO · $150–$350
A failing thermostat, or one wired wrong after a swap, never sends the call for cooling. If the screen works but the AC never responds, the thermostat or its wiring is suspect.
Smart-thermostat C-wire gotchaDIY–PRO · $0–$200
Many smart thermostats need a common (C) wire for steady power. Without one, they drop the cooling call intermittently. A C-wire adapter or a technician fixes it.
Safety switches: the AC that refuses to start
Condensate float switchDIY · $0
A full drain pan or a clogged drain line trips the float switch, and the AC shuts down to keep water off your ceiling — it's protecting the house, not broken. Check for a full pan or water at the indoor unit, wet-vac the outdoor drain stub to clear the line, then reset. A monthly bleach or vinegar tab keeps it clear. Reset only after the cause is cleared.
Pressure switch (high/low)PRO · $225+
A refrigerant fault trips the low- or high-pressure lockout, and the unit refuses to start to protect the compressor. That's a sealed-system diagnosis — confirm the symptoms on our low refrigerant signs page, then call.
Limit switchPRO · Pro
A tripped limit or safety switch on the air handler can lock the whole system out. A technician traces which switch opened and why before resetting it.
Frozen coil & refrigerant causes
Frozen evaporator coilDIY thaw · $0 to thaw
Ice on the indoor unit or lines can trip a safety lockout. Turn the system OFF and the fan ON, and let it thaw 2–4 hours before booking anything — a technician can't test a frozen system, so thawing first saves a wasted visit. If it re-freezes after thawing, the root is airflow or refrigerant.
Refrigerant leak → pressure lockoutPRO · EPA · $225+
A low charge trips the low-pressure switch and locks the compressor out to protect it. This is EPA 608–licensed work, and a recharge without a leak repair just vents again. See low refrigerant signs to confirm, and AC running but not cooling if it starts but blows warm.
What the technician will do
- Check power & voltage — confirm the breaker, disconnect, and voltage at the unit before touching parts.
- Test capacitor & contactor — meter the start components, the most common no-start culprits.
- Read refrigerant pressures — rule a leak-driven lockout in or out with gauges.
- Quote before repair — you get a written price before any work starts.
The visit fee is typically $75–$200 and usually credited toward the repair — details on HVAC service call cost, and a full walkthrough on what to expect from a service call.
When to stop and call
You've done the checks and cleared any tripped safety switch. If it's still dead, the fault is a capacitor, contactor, wiring, refrigerant, or the compressor — all licensed-technician territory. One safety line worth repeating: a capacitor holds a charge even with the power off, so opening the outdoor panel is a real shock hazard. Pricing the fix first? See AC repair cost.
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Frequently asked questions
My thermostat is set to cool but the AC won't turn on — why?
The thermostat is calling for cooling but something downstream is blocking it: a tripped breaker, a pulled outdoor disconnect, a blown fuse, a tripped condensate float switch, or a failed capacitor. Work the 10-minute checks above in order — most cases end at the breaker, disconnect, or float switch.
What's the first thing to check when the AC stops working?
The thermostat, then the breaker, then the outdoor disconnect box. Those three cover the majority of no-start calls and cost nothing to check.
The inside runs but the outside unit won't turn on — what's wrong?
That split points to the outdoor unit specifically: a tripped outdoor breaker or disconnect, a failed capacitor (it hums but won't spin), a bad contactor, or a fan motor. The indoor blower runs on a separate circuit, which is why it still works.
Where is the reset button on my AC?
Most central ACs don't have a single reset button. You reset by cycling the breaker off and on once, or by clearing a tripped safety switch (like the condensate float). If you find a button on the outdoor unit, it's usually a high-pressure reset — a re-trip there means a refrigerant fault, so call.
What is the 3-minute rule?
After power is restored or the thermostat calls for cooling, the compressor deliberately waits up to 3 minutes before starting. This anti-short-cycle delay protects it, so a unit that seems dead may just be counting down — wait the full three minutes.
How do I know if the fuse is blown?
Pull the fuse block from the outdoor disconnect box (power off first) and check the fuses with a multimeter, or look for a scorched or discolored fuse. A blown fuse that blows again immediately means there's a short — replace it once, and call if it goes again.
My AC suddenly stopped mid-day — breaker or capacitor?
On a hot afternoon, a capacitor overheating and failing is a common mid-day cause, and it shows as a hum with no start. A tripped breaker is silent with no hum. Feel and listen at the outdoor unit: hum-but-no-spin points to the capacitor.
Can a dirty filter stop the AC from turning on?
Indirectly, yes. A filter clogged solid collapses airflow, ices the coil, and can trip a safety switch that locks the system out. Swap the filter, let any ice thaw, and it usually restarts.