The AC is running, but the house isn't cooling.

Start with the cheap end of the list. In a Phoenix July, the most common reason a system runs without cooling has nothing to do with refrigerant.

Ten minutes, before you call anyone

Set the thermostat fan to AUTO, not ON. On ON, the blower runs continuously, including between cooling cycles. You get a steady stream of room-temperature air out of the vents and the house feels worse. This one costs nothing and it comes up constantly.

Look at the filter. Hold it up to a light. If you cannot see light through it, it is choking the system. A restricted filter drops capacity, and in a bad case it ices the indoor coil, at which point airflow stops almost entirely.

Walk outside and look at the condenser. The fan on top should be spinning. If the unit is silent while the indoor blower runs, you have a problem on the outdoor side: a tripped breaker, a failed capacitor, or a contactor that is not pulling in.

Look at the coil fins on the outside of the unit. After a monsoon blows through, those fins pack with dust and cottonwood. A condenser that cannot reject heat will run all day and cool very little. If you can see a grey mat across the fins, that is a real cause, not a cosmetic one, and it is what the chemical coil clean in a deep tune-up is actually for.

Check that the return is not blocked. Furniture against a return grille, or half the supply vents closed off in unused rooms, will starve the system.

What it usually turns out to be

In rough order of how often it comes up through a Valley summer:

1. A failed run capacitor. Heat kills them, and Phoenix supplies the heat. The compressor hums and does not start, or the outdoor fan sits still.
2. A fouled condenser coil. Dust in, no heat out. The system runs constantly and holds the house ten degrees above setpoint.
3. A pitted contactor. The relay that switches the outdoor unit on stops making proper contact, so the compressor never gets power.
4. Low refrigerant from a leak. Real, and more expensive to chase, because the leak matters more than the charge.
5. Airflow. Filter, blower wheel caked with dust, or a collapsed flex duct.

What finding out costs

If the system is running but running badly, that is a diagnostic. If it is running normally and you want a full read on where it stands, that is a check-up, which is cheaper. The difference is the symptom, not the amount of work. Both are on the price list, along with every repair that can follow one.

Nothing is credited against a repair, because the diagnosis is the service. You get the readings and the photos either way, and you keep them whether you have the work done here or somewhere else.

When to stop and call

Stop if you see ice on the copper lines or the indoor coil. Stop if the breaker trips a second time after you reset it. Stop if there is a burning smell. None of those get better by running the system harder, and a compressor that floods with liquid refrigerant is a far more expensive morning than a service call.