When an AC won't turn on, the first thing to settle is what "not turning on" looks like: a blank thermostat, a lit thermostat with nothing running anywhere, or an indoor fan that runs while the outdoor unit stays silent. Each of those points to a different part of the system. This page sorts the symptom, covers the case where the thermostat is lit and calling for cooling but nothing starts, and sends the other cases to the guide that covers them in depth.
You set the thermostat to Cool, dropped the setpoint, and waited. Nothing. A central air conditioner is really two machines, one indoors and one outside, that share a single low-voltage control path. Where that path breaks decides what you see, and that is the fastest way to the cause.
- Which "not turning on" do you have?
- How a call for cooling reaches your AC
- Thermostat lit, nothing running: five checks
- After a power outage, a filter change, or a new thermostat
- If only the outdoor unit is dead
- What a no-start repair costs in 2026
- When to stop and call a technician
- Frequently asked questions
Key Takeaways
- A call for cooling is a 24-volt signal from the thermostat, and those 24 volts are made inside the furnace or air handler. Anything that cuts power to the indoor unit cuts the call too.
- A lit screen does not prove the system has power. Battery-powered thermostats stay lit when the furnace is switched off.
- Many thermostats hold the compressor off for up to about five minutes after it last ran, so give a reset that long before deciding it failed.
- A blower door left loose after a filter change can look like a dead system, because a safety switch cuts power until the panel is seated.
- One breaker reset is a fair test. A second trip is a fault for a licensed technician, not a reason to reset again.
Which "not turning on" do you have?
Start with what you can see and hear.
| What you notice | Where the fault usually is | Where to go |
|---|---|---|
| Thermostat screen is blank | No power reaching the thermostat | Thermostat screen blank |
| Thermostat is lit and calling for cooling, but nothing runs anywhere | Settings, the compressor delay, or power to the indoor unit | The five checks below |
| Indoor fan runs, outdoor unit is silent | The outdoor side: disconnect, breaker, contactor or capacitor | AC compressor not turning on |
| Outdoor unit hums, but its fan does not spin | A weak capacitor or a seized fan motor | Outside AC fan not spinning |
| Breaker trips again right after you reset it | An electrical fault | AC breaker keeps tripping |
| Water in the pan or on the floor near the indoor unit | A clogged drain has tripped the safety switch | Condensate safety switch |
| Everything runs, but the house will not cool | Airflow, refrigerant or sizing, not a no-start | AC running but house won't cool |
How a call for cooling reaches your AC
A thermostat is a switch, not a power source. It closes a low-voltage circuit whose 24 volts come from a small transformer inside the furnace or air handler, and that transformer runs on the indoor unit's own household circuit.
When you ask for cooling, the thermostat sends the signal two ways: one wire starts the indoor blower, another tells the outdoor unit to start. The outdoor unit has its own 240-volt breaker and a disconnect box on the wall beside it, but it only starts when that 24-volt signal reaches it.
So the symptom tells you where to look. A break anywhere on the shared path stops everything at once, and the outdoor disconnect cannot cause that on its own. A break on the outdoor side leaves the indoor fan blowing room-temperature air while the unit outside stays quiet. For example, a blower door left ajar stops the indoor fan and the outdoor unit together, while a switched-off outdoor disconnect stops only the unit outside.
Thermostat lit, nothing running: five checks
If the screen is blank, the blank thermostat guide walks the power path step by step. If the screen is lit and still nothing starts, work through these in order.
1. Settings, then wait out the delay
Confirm the mode is Cool, not Heat, Off, or Auto with no cooling setpoint, and set the temperature a few degrees below the room. Then wait. Many thermostats build in a delay after the compressor last ran to protect it; Honeywell describes it as a 5-minute delay (Honeywell Home). A system that seems dead for the first few minutes after you change the setting, or after a reset, may simply be waiting.
2. Is the indoor unit actually powered?
A lit screen is not proof. Battery-powered thermostats stay lit even when the furnace or air handler has no power, so check the indoor unit itself: the service switch on or near it, which furnace makers require at the furnace location (Goodman installation manual) and which looks enough like a light switch to get flipped off by mistake, and the indoor unit's breaker in your panel. A breaker caught in the middle position gets one firm reset, off and then on; a repeat trip is the fault itself, and our guide on why an AC breaker keeps tripping explains what causes it.
3. The blower door
A gas furnace has a door switch that cuts power whenever the blower door is open. Goodman's manual puts it plainly: the interlock switch "interrupts unit voltage when the blower door is opened for servicing. Do not defeat this switch" (Goodman). A panel sitting slightly crooked after a filter change does the same thing. Press it firmly into its slots until it sits flush. Our guide to furnace safety switches explains why this one switch stops heating and cooling together.
4. The drain pan
If water is standing in the pan under the indoor unit, a float switch has probably shut the system down. California's mechanical code calls for a water-level device that "will shut off the equipment or appliance in the event the primary drain is blocked," or a pan or drain alternative, wherever an overflow could cause damage (2025 California Mechanical Code §310.2). Wired into one control wire, the switch stops the whole unit; wired into another, the indoor fan keeps running (RectorSeal Safe-T-Switch data sheet). Clear the drain as our clogged AC drain line guide shows, and never bypass the switch.
5. The control board light
Most modern furnaces report faults by flashing an LED on the control board, and the pattern maps to a code in the wiring diagram or manual (Amana installation manual). If you can see the light without opening the blower door, read it first, because opening the door trips the door switch and the light goes out. No light at all means no power is reaching the board, or, as Goodman's chart also lists, a blown fuse; go back to check 2 first. A board with power but no thermostat call may have blown its low-voltage fuse, 3 amps on many boards, which Goodman's troubleshooting chart ties to a short in the low-voltage wiring (Goodman). Finding that short is a technician's job; a new fuse on an unfixed short blows just as fast.
After a power outage, a filter change, or a new thermostat
A system that worked yesterday and is dead today often has a recent event behind it. Three are worth ruling out first.
After a power outage or a flicker. Check both AC breakers, the indoor one and the larger double breaker for the outdoor unit. Then give the system the full compressor delay before deciding it failed. A smart thermostat also has to reboot and reconnect before it calls for cooling again.
After changing the filter. Look at the blower door before anything else. If your filter slot sits behind that door, a panel left off its hooks holds the door switch open.
After installing a new thermostat. Some systems need a common wire, the C wire, to power a smart thermostat, and Google lists a display that won't turn on among the signs of a power problem (Google Nest Help). A wire that touched the wrong terminal during installation can also short the 24-volt circuit and blow the board fuse. If the old thermostat worked and the new one never has, those are the two places to start.
If only the outdoor unit is dead
If the indoor fan runs but the unit outside never starts, the shared path is fine. Two checks are safe for a homeowner: give the outdoor unit's double breaker its one reset, and make sure the disconnect box on the wall beside the unit is switched on or its pull-out block is seated. Electrical code puts that disconnect within sight of the equipment (EC&M, quoting NEC 440.14).
Past those two, the usual suspects are a failed contactor, a weak capacitor, or the compressor, all on the 240-volt side of the outdoor unit, which makes them a technician's job. Our guide to an AC compressor that won't turn on covers how each one shows up.
What a no-start repair costs in 2026
The five checks above cost nothing. If the fix needs a part or a technician, our $89 diagnostic visit counts toward the repair, and you see the price in writing before we start. The ranges below are national figures as published by HomeGuide (page dated August 22, 2025) and Angi (updated August 3, 2026), not our price list.
| Repair | National range | Source |
|---|---|---|
| AC fuse or breaker replacement | $75 to $300 | HomeGuide |
| Condensate drain line flush | $75 to $250 | HomeGuide |
| Thermostat replacement | $140 to $350 | HomeGuide |
| AC circuit board replacement | $150 to $700 | HomeGuide |
| Contactor replacement | $200 to $450 | HomeGuide |
| Capacitor replacement | $200 to $400 | Angi |
Compressor failures and replacement decisions cost far more; our Sacramento AC repair cost guide covers them.
When to stop and call a technician
- The breaker trips a second time. That is an electrical fault, not a nuisance.
- You smell something hot or burnt near the indoor unit or the panel. Switch the AC off at the panel and leave it off.
- The board fuse blows again after it was replaced. The short is still there.
- The drain pan refills soon after you cleared the line, or water has reached a ceiling.
- The board has no light even though the service switch, the breaker and the blower door all check out.
Alpha Mechanical (California contractor license CSL #967727) handles AC repair across Sacramento from our base in Fair Oaks, including nearby Carmichael and Citrus Heights, with same-day weekday service when the schedule allows. Reach us at 916-848-5980, or book a visit online.
Frequently Asked Questions
Why is my AC not turning on at all, not even the fan?
When the indoor fan stays off too, the break is on the path both halves of the system share: the indoor unit's power, its service switch or breaker, the blower door switch, the board fuse, or a float switch wired to stop everything. If the thermostat screen is blank as well, start with our blank thermostat guide.
Why is my thermostat on but the AC won't turn on?
A lit screen only proves the thermostat has power, and battery-powered models stay lit when the furnace is switched off. Confirm Cool mode and a setpoint below room temperature, wait out the compressor delay of up to about five minutes, then check the service switch, the indoor breaker, the blower door and the drain pan.
Why won't my AC turn on after a power outage?
Check both AC breakers first: a breaker that tripped during the outage stays off when the power comes back. Then wait out the compressor delay, and give a smart thermostat time to reboot. A second trip means a fault, so leave that breaker off and call.
Why won't my AC turn on after I changed the filter?
Check the blower door first. On a gas furnace, a safety switch behind that panel cuts power whenever the door is open or crooked. Press the panel firmly into its slots until it sits flush, and the system should start on the next call for cooling.
Why did my AC stop working after I installed a new thermostat?
The usual suspects are a smart thermostat that needs a C wire in your system, or a wire that touched the wrong terminal and blew the small fuse on the furnace control board. Reconnecting the old thermostat is a quick test; have a technician find any short before the fuse is replaced.
What does it cost when an AC won't turn on?
Often nothing, when the cause is a setting, a switch, a breaker or a loose blower door. Otherwise the $89 diagnostic visit counts toward the repair, and you see the written price before any work. National cost guides put a thermostat replacement at $140 to $350 and a contactor at $200 to $450.