AC running but not cooling the house

Restricted airflow

This is the most common cause and the cheapest to fix. A clogged filter, a dirty evaporator coil or a blocked return starves the system of air. Less air across the coil means less heat removed, and in bad cases the coil gets cold enough to freeze, at which point cooling stops entirely. If your filter is overdue, start there. If it has been changed recently and the problem persists, the restriction is further in. See frozen coils and ductwork.

Low refrigerant from a leak

A system low on charge cannot absorb heat properly. Symptoms are long run times, a small temperature drop across the coil, and often ice forming on the suction line near the outdoor unit. Refrigerant is not consumed in normal operation, so a low system has a leak somewhere. Adding refrigerant without finding the leak is a temporary measure that has become substantially more expensive since the R-410A phase-down began. See refrigerant leak repair.

A failing capacitor

A capacitor losing capacitance may still start the outdoor fan while leaving the compressor struggling or not running at all. The result is an outdoor unit that appears to be working, with the fan spinning, while no actual cooling is happening. This is a common and inexpensive fix. See capacitor replacement.

A dirty condenser coil

The outdoor coil is how heat leaves your house. Coated in grass clippings, dust and cottonwood, it cannot reject heat efficiently, operating pressures climb and capacity falls. This shows up first as a system that copes in the morning and gives up in the afternoon, which is exactly the pattern people describe most often here.

Duct leakage into the attic

If the ducts leak, cooled air is being delivered to an attic rather than to rooms, and on the return side the system may be pulling in superheated attic air. In a metro that sees around 102 days a year above 90°F, with attic temperatures far above outdoor air, this is a large loss. It usually presents as a system that runs constantly, never quite satisfies, and costs a fortune to run. See ductwork.

A compressor that is not doing useful work

The least welcome answer. A compressor that has failed internally may still hum and draw current without compressing refrigerant. Pressures tell the story quickly. On an older system this is usually the point where replacement enters the conversation. See compressor replacement.