When your AC runs but the house stays warm, the most common causes are a thermostat set wrong, a clogged air filter, a tripped breaker for the outdoor unit, or ice on the indoor coil. Bigger problems like a refrigerant leak, a failed capacitor, or a dirty coil need a technician. Try the safe checks below first, then call for service if the air still is not cool.
Start with the easy checks
Many no-cooling calls end up being a setting or a switch. Check these before anything else:
- Thermostat mode: Make sure it is set to Cool, not Heat or Off, and that the set temperature is a few degrees below the room temperature.
- Fan setting: If the fan is set to On, it blows air even when the AC is not cooling. Set it to Auto so you can tell when cooling is actually running.
- Batteries: A blank or dim thermostat screen often means dead batteries.
- Breakers: The outdoor unit has its own breaker in your main panel. If it tripped, the indoor fan may run while the outdoor unit sits idle, so the air feels warm. Reset it once. If it trips again, leave it off and call a pro.
Check the air filter
A clogged filter is one of the top reasons an AC runs and runs without cooling. It chokes off airflow, so the system cannot move enough air across the cold coil. Pull the filter out and hold it up to a light. If you cannot see light through it, replace it. ENERGY STAR suggests checking your filter once a month and changing it when it looks dirty. Homes with pets, dust, or heavy summer use often need changes more often.
Look at the outdoor unit
On a hot day, walk outside while the system is running.
- Is the top fan spinning? If the fan is still, or you hear a hum but nothing turns, a part such as the capacitor or motor may have failed. Turn the system off at the thermostat so the compressor does not overheat, and call for repair.
- Is there room to breathe? Keep plants, fences, grass clippings, and stored items at least a couple of feet away so the unit can release heat.
- Do the fins look matted or dirty? A dirty outdoor coil cannot get rid of heat well. Coil cleaning is part of a normal tune-up.
Do not open the electrical panel on the unit. It holds high-voltage parts that stay charged even after you turn the system off.
Check for ice and water
Look at the copper lines going into the outdoor unit and near the indoor coil above your furnace. Frost or ice means the coil is freezing, which blocks airflow and stops cooling. Turn cooling off and set the fan to On to let it thaw. Our guide on ice on AC lines or the coil explains what to do next.
Also look at the drain line and any condensate pump near the furnace. If water is backing up, a float switch may shut down the outdoor unit to prevent a leak. You can look for standing water, but leave cleaning out the drain and pump to a technician if it is not obvious.
Problems that need a technician
If the easy checks do not fix it, the cause is usually inside the equipment:
- Low refrigerant from a leak. An AC does not use up refrigerant. If it is low, there is a leak to find and fix. Federal rules require technicians who service equipment that could release refrigerant to be EPA Section 608 certified, so this is not a do-it-yourself job.
- Failed capacitor or contactor. These parts start the outdoor fan and compressor. They wear out, especially after heat waves.
- Dirty indoor coil or weak blower. Both cut airflow and cooling.
- Leaky or crushed ducts. Cool air may be escaping into the attic or walls before it reaches your rooms.
- A system that is too small. If your AC has never kept up on the hottest days, the size may be the problem. See what size AC your house needs.
When to call a pro
Call for service if the outdoor unit will not run, a breaker trips more than once, you see ice that comes back after thawing, you hear buzzing or grinding, or the air from your vents still feels warm after you changed the filter. Yogi Heating & Cooling handles AC repair across Schaumburg and nearby suburbs. Call (814) 431-1709 or book a visit online. A spring AC tune-up catches many of these problems before the first hot week.
More questions
Should I keep running my AC if it is not cooling?
No. Turn cooling off at the thermostat. Running it with ice on the coil, a stuck outdoor fan, or low refrigerant can damage the compressor, which is one of the most expensive parts to replace.
Can I add refrigerant myself?
No. Refrigerant work requires EPA Section 608 certification and proper tools. Adding refrigerant without fixing the leak also just delays the next breakdown.
How much cooler should the air from my vents be?
Many technicians look for supply air roughly 15 to 20 degrees cooler than the air going into the return. A much smaller difference points to a problem worth checking.
Why does my AC cool at night but not in the afternoon?
A dirty outdoor coil, low refrigerant, or a system that is too small can keep up when it is mild but fall behind in peak heat. A technician can test which one it is.
Related
- AC repair and installation
- Why is there ice on my AC lines or coil, and what should I do?
- Maintenance plan
- Should you repair or replace your AC or furnace?