When an AC won't turn on after sitting idle all winter, the cause is usually simple: the thermostat isn't asking for cooling, something in the power supply is switched off, or a safety switch is holding the system off. Failed parts and refrigerant problems happen too, but they're further down the list. Work through the checks below in order, from the thermostat inside to the condenser outside, and you'll either find the problem or know exactly what to tell a technician.
Before you start, notice what is and isn't running. If nothing happens at all, not even the indoor fan, look first at the thermostat and the furnace or air handler. If the indoor blower runs but the outdoor AC unit isn't running, its power supply and controls are the likely suspects. If the outdoor unit hums but its fan doesn't spin, skip ahead to the problems that need a technician.
1. Start at the thermostat
It sounds too obvious to matter, but a thermostat still set to heat from the winter is easy to overlook. Set the mode to Cool, set the temperature a few degrees below what the room is now, and leave the fan on Auto. If you use a schedule or an energy-saving hold, make sure it isn't overriding your setting.
A blank or dim screen points to power. If your thermostat takes batteries, put in fresh ones. Many smart thermostats instead draw power through a common wire (the C-wire) from the furnace or air handler, so if that equipment is switched off, the thermostat goes dark too, which leads straight to the next checks. Also give it a few minutes: many thermostats have a built-in delay that keeps the outdoor unit from starting right after a setting change.
Wait for a genuinely warm day to test. Air conditioners aren't designed to run in cooling mode when it's cool outside; one major manufacturer's installation instructions, for example, list 55°F as the lowest outdoor temperature for cooling without a low-ambient accessory.
2. Check the breakers and the outdoor disconnect
A central air system usually has two circuits: one for the furnace or air handler indoors and a separate, larger breaker for the outdoor condenser. Open the door of your electrical panel (just the door, never the cover behind it) and look for a breaker sitting in the middle or off position. Reset a tripped breaker once by pushing it fully off and then back on. If it trips again, stop there and call for service, because a breaker that keeps tripping is protecting you from a real fault.
Next, find the disconnect: the small box on the outside wall near the condenser. Some homeowners, or the person who closed up the system in the fall, pull it for the winter. Set the thermostat to Off first. If the box has a pull-out block, make sure it's in place and inserted in the On position (the markings are usually printed on it); if it has a lever, make sure it's up or on. If the disconnect holds fuses or anything inside looks burned or damaged, leave it to a technician.
If the outdoor unit was without power all winter
Your outdoor unit may have a crankcase heater, a small heater that keeps the compressor's oil warm so liquid refrigerant doesn't settle in it while the system sits. Some manufacturers' instructions call for power to the outdoor unit for as long as 24 hours before the compressor starts, with the thermostat set to Off, so that heater has time to work. Not every unit has one and the instructions vary by model, so check your installation manual (it's usually available online by searching the model number on the unit's data plate). If the breaker or disconnect was off all winter, restoring power a day before you need cooling is an easy way to follow that advice when it applies.
3. Look at the furnace or air handler
Your air conditioner depends on the same indoor blower that moves air for your heat, so the indoor unit has to be powered and ready. Look for a service switch on or near the furnace; it looks like an ordinary light switch and is sometimes mounted at the top of the basement stairs, where it gets flipped off by mistake. Make sure it's on.
Then check the blower compartment door. Furnaces and air handlers have a door safety switch that cuts power whenever that panel is off or not seated properly. If someone changed the filter or looked inside over the winter and the door went back slightly crooked, nothing will run. Press it firmly into place. Many furnaces also have a small status light visible through a window in the door, and the code chart on the panel tells you what a blinking pattern means.
4. Check the condensate overflow switch or pump
An air conditioner pulls water out of the air, and that water drains away through a pipe or tubing from a pan under the indoor coil. To prevent overflows, most systems have a float switch in the pan or drain line that shuts the cooling off when water backs up. If your system shut down mid-season last year and was never fully looked at, a clogged drain may still be holding it off.
Look for standing water in the drain pan or around the base of the furnace or air handler. If your system uses a small condensate pump, check that it's plugged in and that its reservoir isn't full of water that never pumped out. Clearing the blockage is what fixes it; a float switch that trips again a week later is telling you the drain is still partly clogged.
5. Change the filter
A badly clogged filter rarely stops an air conditioner from starting, but it can stop it from working. Restricted airflow lets the indoor coil get too cold and ice over, and a coil buried in ice barely cools, then melts into more water than the drain may handle. A filter that made it through a full heating season is due anyway, so put in a fresh one with the arrow pointing toward the furnace. Our guide to how often to change your furnace filter covers the schedule year-round.
6. Clear the outdoor unit
Take off any winter cover before the first run. Covers that wrap the whole unit can trap moisture and give mice a sheltered spot to nest, so if you use one at all, a top-only cover is the gentler choice. Then look the unit over:
- Clear leaves, sticks and grass clippings from the top grille and around the base. With the power off at the disconnect, you can rinse dirt from the coil fins with a garden hose, but never a pressure washer.
- Cut back shrubs, tall grass and anything else crowding the unit. Required clearances vary by model; one major manufacturer, for example, asks for at least a foot on most sides and four feet above the fan.
- Check that the unit still sits level on its pad and that the refrigerant lines and wiring weren't damaged over the winter by snow, a plow, a string trimmer or animals. Don't touch damaged wiring; that's a technician's job.
Problems that need a technician
If you've been through the checklist and the system still won't run, the cause is probably a part or a fault that shouldn't be handled by a homeowner. These are the usual suspects:
A failed capacitor. The capacitor gives the compressor and outdoor fan motor the kick they need to start. When it fails, the outdoor unit typically hums but the fan doesn't spin, or the compressor tries to start and clicks off. Capacitors can hold a dangerous electrical charge even with the power off, so leave this one alone.
A bad contactor. The contactor is the relay that switches the outdoor unit on when the thermostat calls for cooling. Pitted contacts or insects nesting inside can keep it from closing, so the indoor blower runs while the outdoor unit sits silent.
A blown low-voltage fuse. Many furnace control boards have a small fuse that protects the thermostat circuit. It usually blows because a thermostat wire is shorted somewhere, often the cable running out to the condenser, and replacing the fuse without finding that short just blows the new one.
Refrigerant problems. An air conditioner doesn't use up refrigerant, so if it's low, there's a leak. Some systems shut down on a low-pressure switch; others run without cooling much. Under EPA rules, only Section 608 certified technicians can buy refrigerant for home air conditioning, and the right fix is finding and repairing the leak, not just topping it off. Never try to add refrigerant yourself.
Electrical warning signs. A breaker that trips again after one reset, scorch marks, melted insulation or a burning smell all mean turning the system off at the thermostat and the breaker and getting it checked before running it again.
If it runs but doesn't cool
Sometimes the system starts fine but the air from the vents is barely cool. That's a different set of causes, from a dirty outdoor coil to low refrigerant or a frozen indoor coil, and our guide to an AC that runs but isn't cooling walks through them. And if you'd rather have the whole system looked over before the first heat wave, our HVAC maintenance checklist shows what a proper spring tune-up covers.
If the AC still won't start after these checks, or you've landed on one of the technician problems above, you can book an AC repair visit. It helps to mention what you've already checked, and you'll know the price before any work starts.
Sources: EPA, Section 608 refrigerant sales restriction; example manufacturer installation instructions for a residential air conditioner (crankcase heater, clearances and minimum cooling temperature).