Why Is My AC Running but Not Cooling? 7 Common Causes (and What They Cost to Fix)

It’s 95° in San Jose, your AC is running non-stop, and the house isn’t getting any cooler. Before you panic (or sweat through another night), here are the seven causes we find most often on service calls — starting with the ones you can check yourself.

1. A clogged air filter — check this first (free)

A packed filter chokes airflow, and low airflow can even freeze the indoor coil. Pull your filter and hold it up to a light: if you can’t see light through it, replace it, set the fan to ON to thaw any ice, and give the system a few hours.

2. Thermostat settings or dead batteries (free–$)

It sounds silly until it’s the fix: make sure it’s on COOL, the setpoint is below room temperature, and the batteries are fresh. A blank or unresponsive thermostat can also mean a tripped float switch — which brings us to…

3. Clogged condensate drain (~$150–$300)

Your AC pulls gallons of moisture out of the air, and that water leaves through a small drain line. When it clogs, a safety switch shuts the system down to prevent water damage. Quick fix for a technician.

4. Weak capacitor (~$150–$400)

The most common electrical failure in hot weather. The capacitor gives your outdoor unit’s motors their starting kick; when it weakens, the fan or compressor won’t spin up — the system hums but doesn’t cool. Cheap part, fast fix.

5. Dirty condenser coil (~$150–$350 as part of a tune-up)

The outdoor unit needs to dump heat. Coated in dust, cottonwood fluff, or dryer lint, it can’t — and cooling capacity plummets while your bill climbs. This is exactly what an annual tune-up prevents.

6. Refrigerant leak (~$400–$1,500+)

Low refrigerant from a slow leak means warm air, ice on the lines, or hissing sounds. The refrigerant doesn’t “run out” on its own — there’s always a leak to find and fix. Beware anyone who just “tops it off” every summer without finding the leak.

7. Failing compressor (repair vs. replace decision)

The big one. If the compressor is dying in a 12+ year-old system, replacement is usually smarter than a $2,500+ repair — especially with rebates on heat pumps that cool and heat. We’ll give you both numbers honestly.

When to stop troubleshooting and call

If the filter and thermostat check out and it’s still blowing warm — or you see ice anywhere — shut the system off and call (669) 777-1997. Running a struggling AC destroys compressors. We offer same-day service across San Jose 7 days a week, and the service call is free with any repair.