What is your AC doing?
Match what you see to the table below, then work through the safe checks further down the page.
| Symptom | Common causes | Check first |
|---|---|---|
| Running but not cooling | Clogged filter, frozen indoor coil, refrigerant leak, dirty outdoor coil | Filter, and whether the outdoor unit runs |
| Blowing warm air | Outdoor unit not running: tripped breaker, failed capacitor or contactor | Thermostat on COOL, breaker, outdoor disconnect |
| Not turning on at all | Thermostat, tripped breaker, blown fuse, condensate float switch | Thermostat batteries, breaker, drain pan |
| Outdoor unit not running | Capacitor, contactor, disconnect switched off, fan motor | Breaker and disconnect, listen for a hum |
| Ice on the lines or indoor coil | Low airflow, refrigerant leak, blower problem | Turn cooling off, check the filter |
| Water leaking indoors | Clogged condensate drain, full drain pan, thawing coil | Turn the system off, look at the drain pan |
| Short cycling | Clogged filter, frozen coil, low refrigerant, thermostat location, overheating compressor | Filter and thermostat settings |
| Buzzing, grinding or squealing | Capacitor or contactor (buzz), worn motor bearings (grinding, squeal), loose parts (rattle) | Turn it off and call |
Frozen lines or ice on the indoor coil
Ice on the copper lines or the indoor coil means the coil got colder than it should. The usual reasons are weak airflow (dirty filter, blocked returns, failing blower) or low refrigerant from a leak. Turn cooling off at the thermostat and set the fan to ON to help it thaw. Don't chip at the ice. If it freezes again, it needs a pro.
Water leaks
Your AC pulls moisture from the air and drains it through a condensate line, which algae can clog. Many systems have a float switch that shuts the AC off when the drain pan fills, so a clogged drain can look like an AC that won't turn on. If you see water in the pan or on the floor, turn the system off and call.
Short cycling
Short cycling means the AC turns on and off every few minutes without cooling the house, which wears out the compressor fast. Common causes are a clogged filter, a frozen coil, low refrigerant, a thermostat near a sunny window or supply vent, or an overheating compressor. An oversized system can do it too.
Noises
A steady hum is normal. Loud buzzing outside often points to a failing capacitor or contactor. Grinding or squealing usually means worn motor bearings, hissing can mean a refrigerant leak, and banging means something is loose. Turn the system off and have it checked.
Safe checks before you call
These checks take a few minutes and need no tools. Even when they don't solve the problem, they give the pro useful information.
- Thermostat on COOL, fan on AUTO. Set it a few degrees below room temperature. With the fan on ON, the blower runs even when the AC isn't cooling, which feels like warm air. Replace weak batteries.
- Check the air filter. If you can barely see light through it, replace it. A clogged filter is a common cause of a frozen coil and weak airflow.
- Check the breakers. Central AC usually has two: one for the indoor unit (furnace or air handler) and a double breaker for the outdoor unit. If one has tripped, switch it fully off and back on, one time. If it trips again, leave it off and call.
- Check the outdoor disconnect. This small box on the wall near the outdoor unit has its own switch. If it has a lever, make sure it's set to ON. If it's a pull-out type and you aren't sure, leave it alone and tell the pro.
- Clear around the outdoor unit. Remove leaves, grass clippings and branches, and keep about 2 feet of clear space on every side. Nothing should sit on the unit.
- Open vents and returns. Make sure supply vents are open and nothing blocks the return grilles.
If a baby, an older adult or someone with a health condition is at risk from the heat, move them somewhere cooler while you wait.
Common causes the pro will look at
When the simple checks don't solve it, these are the parts a pro usually checks first. All of them are jobs for a technician.
Capacitor
The capacitor gives the compressor and fan motor the boost they need to start. When it fails, the outdoor unit often hums while the fan doesn't spin, or it won't start at all.
Contactor
This switch in the outdoor unit turns the compressor and fan on when the thermostat calls for cooling. Burned contacts or insects caught inside can keep it from closing or leave it stuck on.
Refrigerant leak
Refrigerant isn't used up like fuel, so a system that's low has a leak. Signs include warm air, long run times, ice on the coil and hissing. A proper repair finds and fixes the leak before recharging.
Dirty coils
A dirty indoor (evaporator) coil can't absorb heat well and can freeze. A dirty outdoor (condenser) coil can't get rid of heat, so the system runs hot and long.
Condensate drain and float switch
A clogged drain backs water up into the pan. The float switch then shuts the AC off to prevent a flood. A pro clears the line and finds out why it clogged.
Blower motor
The indoor blower moves air across the coil. A failing blower means weak airflow, a coil that freezes and rooms that never cool down. Grinding or squealing indoors often comes from here.
Independent AC pros commonly work on most makes and models of central air conditioners and heat pumps. If your system also heats your home, see heat pump repair. To learn what happens when you call, see how it works.
