Learning Center / Repair
Learn · 6 min read
Why Isn't My AC Cooling? — Six Real Causes, Ranked by How Often I See Them
In our first summer at TCS I kept a service-call tally. Six causes accounted for 91% of "AC not cooling" calls in the the Triangle. Here they are in order of frequency — with what you can check yourself in the first five minutes.
What does 'not cooling' actually mean?
Diagnostic precision matters. "Not cooling" splits into three symptoms with very different causes:
- Air is blowing but it's not cold.
- Air is barely blowing at all.
- Outdoor unit isn't running.
The 30-second homeowner check: put your hand at the supply register and the return. If supply feels close to return temperature, the compressor isn't moving heat. If supply is cold but weak, it's an airflow problem.
The six causes, ranked
- Failed capacitor (28% of calls). A $30 part; a $200–$300 fix. Symptom: outdoor fan tries to spin, hums, gives up.
- Frozen evaporator coil (19%). Caused by a dirty filter or low refrigerant. Symptom: air barely blowing, ice visible on the copper lines outside.
- Refrigerant undercharge (15%). Small leak, usually at a Schrader valve or a rubbed line. Symptom: gradual loss of cool over weeks.
- Thermostat / control issue (12%). Dead batteries, misconfigured heat pump balance point, bad wiring. Free-to-cheap fix if caught early.
- Dirty condenser coil (11%). Outdoor coil coated in cottonwood fluff, cut grass, dryer lint. Cleaned in 20 minutes.
- Failed contactor (6%). Pitted contacts prevent 24V signal from closing the compressor circuit. $200 fix.
How you fail this diagnosis
The most expensive mistake: pouring more refrigerant into a leaking system. Refrigerant doesn't get "used up." If your system's low on charge, there's a leak, and adding refrigerant without finding the leak means you'll be back to warm air in 3–8 weeks — and paying again.
Frequently asked questions
Why is my AC running but not cooling the house?
Nine out of ten times it's one of: dirty filter (frozen coil), low refrigerant charge, a failed capacitor, or a thermostat set to 'fan' instead of 'cool'. Check the filter first.
Should I add refrigerant to my AC myself?
No. Refrigerant sales to non-EPA-certified individuals are federally restricted, and topping off a system without finding the leak is a $200 mistake you'll repeat every few months.
Action