Start With the Two Safe Checks
Before assuming the worst, check the filter and the outdoor unit. A filter so clogged it bows inward can restrict airflow enough to freeze the evaporator coil, and a frozen coil stops cooling entirely. Outside, look at the condenser: is the fan spinning when the system calls for cooling, and is the unit clear of cottonwood fluff, grass clippings, and vegetation? A smothered condenser cannot dump heat, and cooling suffers in direct proportion. If either check reveals an obvious problem you can fix (new filter, cleared debris), give the system an hour, with any ice fully melted first, and see whether cooling returns.
If Ice Is Involved, Turn the System Off
Ice on the refrigerant line or the indoor coil is a stop sign. Running a frozen system can destroy the compressor, turning a modest repair into the most expensive one on the menu. Shut the cooling off, run the fan-only setting if you want to speed thawing, and call once it has thawed. Ice means either an airflow problem or a refrigerant problem, and both need professional diagnosis.
The Causes a Contractor Will Check
Refrigerant leaks
Refrigerant circulates in a sealed loop; it is not consumed like fuel. If the charge is low, it leaked, and the correct repair is finding and fixing the leak, not just topping off. Symptoms include gradual cooling decline, hissing sounds, and ice formation.
Capacitors and contactors
These inexpensive electrical components start and run the compressor and fan, and they are among the most common AC failures, especially after storms and during the season's first heat. Symptoms: the outdoor unit hums but does not start, clicks repeatedly, or does nothing at all.
Compressor problems
The compressor is the heart of the system and the most expensive component. Hard starting, overheating, and electrical failure all present as "runs but not cooling" or "will not run at all." Compressor diagnosis determines whether you are facing a repair or a replacement conversation.
Dirty coils
A condenser coil matted with debris or an evaporator coil coated in grime cannot move heat effectively. This is the gradual-decline scenario: the system cools, just worse every month. See AC cleaning for what maintenance addresses.
Thermostat and control issues
Sometimes the equipment is fine and the signal chain is broken: thermostat settings, dead batteries, wiring faults, or a failed control board. Cheap to fix when that is the cause.
When to Call
Call when the safe checks do not restore cooling, when ice appears, when the outdoor unit will not run, or when the breaker trips. Describe symptoms precisely: "the outdoor fan spins but the air is warm" points a contractor toward different suspects than "nothing outside runs at all." The AC repair page covers the repair process, and emergency HVAC explains urgent availability during heat waves.