The condenser.
The outdoor unit. Its whole job is throwing your house's heat into the air, which gets harder at 115.
When somebody says “the AC unit” and points outside, they mean the condenser. It holds the compressor, the condenser coil wrapped around the outside, the fan on the top deck, and the electrical compartment with the contactor and capacitor in it.
Its job is simple and unforgiving: take the heat your house just gave up and throw it into the outdoor air. On a 115 degree afternoon it is trying to dump heat into air that is already hot, which is why Valley systems run so much longer than the same equipment would anywhere else.
The fins are not cosmetic
The coil is a grid of thin aluminium fins around copper tubing, and all the heat transfer happens across them. Pack them with dust and the heat has nowhere to go. Head pressure climbs, run times stretch, the power bill goes up, and the house still does not get comfortable.
After a monsoon season those fins hold a grey mat that a garden hose moves the surface of and nothing else. That is the difference between a rinse and a chemical clean, and both are on the price list.
Clearance
The unit rejects heat sideways and upward. Oleander that grew into it over the winter, a trellis somebody added, bags leaned against it in the side yard: all of it recirculates hot air back through the coil. Two feet clear all round, and nothing over the top.
What usually fails in it
In rough order through a Phoenix summer: the capacitor, the contactor, the fan motor, the coil being filthy, and then the compressor. The first four are all flat priced and all well inside the one thousand dollar ceiling.