Home › Troubleshooting › AC won't turn on at all? Seven checks before you call
AC won't turn on at all? Seven checks before you call
A completely dead AC feels like the worst-case scenario, but the most common causes are a $0 fix: a blank thermostat, an upside-down disconnect, or a clogged drain's safety switch doing its job. Work these in order.
Guide by Zach Barbre · Denver HVAC expert
Updated October 2026 · Written from first-hand field experience in the north Denver metro
First: what does "won't turn on" look like at your house?
- Thermostat display is blank — power or batteries. Check 1.
- Indoor fan runs, outdoor unit is dead silent — power to the condenser, the disconnect, or the capacitor/contactor. Checks 2, 3, and 6.
- Outdoor unit hums or clicks but won't start — capacitor signature. Check 6.
- Everything worked yesterday, nothing works after a storm or outage — check 2, then check 5 (safety float switch), then give it time: many thermostats enforce a compressor-protection delay of up to five minutes after power returns.
The checks, in order
1. The thermostat: display, batteries, settings
Blank screen on a battery-powered thermostat is exactly what it looks like — replace the batteries. On a powered thermostat, a blank screen can mean the furnace's control fuse or the condensate float switch (check 5) has cut power. Confirm COOL mode, a setpoint below room temperature, and — if you just restored power — wait a full five minutes for the compressor-protection delay before concluding anything.
2. Breakers: the panel, then the outdoor disconnect
Check the AC breaker in your panel (it may be labeled condenser, AC, or paired with the furnace). Then go outside: there's a disconnect box on the wall near the condenser. Many are pull-out style, and the pull-out block gets reinserted upside-down after winter or service — which is the OFF position, by design. If it has a handle, confirm it's ON. Reset a tripped breaker once only; a second trip is a fault to diagnose, not a nuisance.
3. The furnace/air-handler switch and door
The indoor unit has its own power switch (light-switch style, often on the unit or a nearby wall) and a blower-door safety switch. If the door was opened for a filter change and not reseated, the whole system — AC included — stays dead. Reseat it and retry.
4. The filter and the blower
Set the thermostat fan to ON. If the indoor blower doesn't run at all, the problem is indoor power or the blower itself, and the outdoor unit is innocent. If the blower runs on ON but the system won't cool on AUTO, you're back to thermostat or outdoor causes. A severely clogged filter can also trip safeties on some systems — swap it while you're there.
5. The condensate float switch (the sneaky one)
Most modern installs have a safety float switch on the AC's condensate drain: when the drain clogs with algae and sludge, the float rises and kills the system before water overflows into your ceiling or basement. Look at the white PVC drain line near the indoor coil — if there's a small switch with wires on a cap, and the drain pan or line is full of water, you've found it. Clearing the clog (a wet vac on the outside end of the drain line pulls most of them through) and emptying the pan usually restores operation. If your system has a condensate pump instead, check that it's plugged in and its reservoir isn't full and stuck.
6. Capacitor and contactor: recognize them, don't open them
If the outdoor unit hums for a few seconds and gives up, or clicks repeatedly, the failed part is usually the capacitor ($150–$350 installed) or the contactor — the two most common no-start repairs, per our cost guide's repair table. This is where the DIY road honestly ends: a capacitor can hold a charge with the power off and punish curiosity. Note the symptom ("it hums for five seconds, then clicks off") and tell the tech — you'll get a faster, cheaper visit for it.
7. After a power outage: check for a surge casualty
If the failure started with a storm or outage, and breakers, disconnect, thermostat, and float switch are all fine, the control board in the furnace or the thermostat itself may have taken the surge. That's diagnostic-visit territory ($75–$150), and it's also the cheapest argument for a surge protector on the HVAC circuit at the next visit.
What it costs if a part has failed
| Common no-start repair | Typical range | Notes |
|---|---|---|
| Service / diagnostic visit | $75–$150 | Often applied to the repair if you approve it on the spot |
| Run / start capacitor | $150–$350 | The most common no-start fix of all |
| Control board | $300–$800 | Model-specific; the usual suspect after a surge |
| Blower motor | $400–$900 | ECM variable-speed motors cost far more than fixed-speed |
If the system turns out to be at replacement age, the Denver-metro cost guide has the installed ranges before you talk to anyone.
Stop and call instead if…
- A breaker trips a second time, or you find scorch marks or a melted disconnect — electrical faults don't get third chances.
- The condensate pan is overflowing and you can't clear the drain — water damage outruns HVAC damage fast.
- The outdoor unit hums and strains repeatedly. Every failed start is hard on the compressor; switch the system OFF at the thermostat until the tech comes, rather than letting it try all day.
Questions homeowners ask about this
Why did my AC stop working after a power outage?
Three usual suspects: a tripped breaker or a disconnect that needs resetting, the thermostat's compressor-protection delay (give it five full minutes), or a surge-damaged control board or thermostat. Work checks 1 and 2 in this guide; if power is present everywhere and it still won't start, it's the third one, and that's a diagnostic visit.
What is the float switch and why did it kill my AC?
It's a safety switch on the condensate drain. Air conditioners pull gallons of water out of the air; if the drain clogs, the rising water lifts a float that shuts the system down before the pan overflows into your home. It's doing its job — clear the drain line (a wet vac on the exterior end usually pulls the clog out) and the system restarts.
My AC hums but won't start. Can I push the fan to get it going?
That hum is the classic failed-capacitor symptom, and spin-starting the fan with a stick does sometimes 'work' — it's also how homeowners get hurt, and it does nothing for the compressor half of the same capacitor. Switch it off and get the capacitor replaced: $150–$350 installed, usually same-visit.
The thermostat screen is blank and there are no batteries. Now what?
A powered (common-wire) thermostat going blank means it lost power from the furnace — check the furnace switch, the blower door, the small automotive-style fuse on the furnace control board, and the condensate float switch, which cuts power when the drain backs up. If none of those restore it, the board itself needs a tech.
Rather have a pro just handle it?
We connect Denver-metro homeowners with licensed local installers — free second opinions, written quotes, no pressure. Tell them what you found in the checks above and you'll skip half the diagnosis.
Typical response time: under 15 minutes, 7am–7pm daily.