Why Is My AC Not Cooling?

Start With the Air Filter

I'd estimate 30-40% of the "AC not cooling" calls I went on were solved by changing the air filter. A clogged filter restricts airflow across the evaporator coil, which means the system physically can't move enough air to cool your house — even though it's running fine mechanically.

Pull the filter out and hold it up to a light. If you can't see through it, that's your problem. Replace it and give the system 30 minutes to recover.

Check the Outdoor Unit

Go outside and look at your condenser (the big box with the fan). Is the fan spinning? If the outdoor unit isn't running at all, check:

• The breaker — the outdoor unit has its own dedicated breaker. Flip it off, wait 30 seconds, flip it back on.
• The disconnect box — there's a gray box on the wall near the outdoor unit. Open it and make sure the pull-out switch is fully seated.
• The capacitor — if you hear a humming sound but the fan won't spin, the run capacitor is likely dead. This is a $15 part but involves working near high voltage — call a tech unless you're comfortable with electrical.

If the fan IS running but the air coming off the condenser isn't hot, that suggests a compressor or refrigerant issue.

Look for Ice on the Indoor Coil

Open the panel on your air handler or furnace and look at the evaporator coil. If it's covered in ice, you have one of two problems:

1. Low airflow — usually the filter (see above), but could also be closed vents, a collapsed duct, or a failing blower motor
2. Low refrigerant — a leak somewhere in the system is causing the remaining refrigerant to expand too much, dropping the coil temperature below freezing

If the coil is frozen, turn the system OFF and let it thaw completely before calling a technician. Running a frozen system can kill the compressor — and that's a $2,000-$4,000 repair.

Thermostat Issues

This sounds basic, but I've walked into plenty of homes where:

• The thermostat was set to HEAT instead of COOL
• The fan was set to ON instead of AUTO (which means it blows room-temperature air between cooling cycles, making it feel like the AC isn't working)
• The temperature was set higher than the room temperature
• The batteries in the thermostat were dead

Check all four. Set it to COOL, AUTO, and at least 3 degrees below the current room temperature. Wait 5 minutes for the system to kick on.

Refrigerant Issues

If none of the above fixes it, you likely have a refrigerant problem. Your AC doesn't "use up" refrigerant — if it's low, that means there's a leak somewhere. Signs of low refrigerant:

• The supply air feels cool but not cold (should be 15-20 degrees colder than return air)
• The larger copper line going to the outdoor unit is sweating or frosted
• The system runs constantly without reaching the set temperature

This requires a licensed technician with gauges and a leak detector. Expect $150-$400 for a leak search and repair, plus $50-$150 per pound of refrigerant.

When to Call a Professional

Call a technician if:

• The outdoor unit won't start even after resetting the breaker
• You see ice on the coil or copper lines and the filter was already clean
• The system runs but the air temperature from the vents isn't at least 15 degrees cooler than the room
• You hear unusual sounds — grinding, screaming, or clicking from the outdoor unit
• You smell something burning

What to tell them: "My AC is running but not cooling. I've checked the filter and thermostat. The outdoor unit [is/is not] running. [I do/don't] see ice on the coil." This saves them diagnostic time and you money.

Expect to pay: $85-$150 for a diagnostic visit, plus parts and refrigerant if needed. A typical "AC not cooling" repair runs $150-$600 total.