Every room is fine except one.

If one room is hot and the rest of the house is comfortable, the air conditioner is almost certainly fine. Something between the system and that room is the problem, and most of the candidates are cheap.

Start in the room

Is the supply vent open, and is air actually coming out? Hold your hand to it. Compare it against a vent in a room that is comfortable. A noticeable difference tells you it is a distribution problem rather than a capacity one.

Is furniture over the vent or the return? A sofa across a floor register or a bookcase against a return does more damage than people expect.

Is the damper closed? Many supply registers have a lever that closes the vane. Somebody closes one in winter and nobody opens it in May.

Then look at the room itself

A Phoenix west-facing room with a big window is working against a different load than an interior bedroom. So is a room over a garage, a converted patio, or anything with a vaulted ceiling.

If the room has always been the hot one, that is a load problem and no part replacement will change it. If it used to be fine and now is not, something changed, and that is worth chasing.

Then the duct

This is where most genuine single-room problems live, and it is almost always in the attic.

A crushed or kinked flex duct. Somebody stood on it, or stored boxes on it, or it has sagged over a joist for twenty years.

A disconnected duct. Flex duct pulls off a boot more often than you would hope, and then you are air conditioning your attic at considerable expense. A room that went from fine to terrible suddenly is the classic sign.

A duct that is simply too long or too small for the run it serves.

A stuck zone damper, if the house has zoning. A damper motor that has failed closed will starve a whole zone while everything else works.

And the return

Supply gets the attention; return is half the system. A room with a big supply and no path for air to get back out will not cool properly. A closed door can be enough on its own, which is why undercut doors and transfer grilles exist.

What a visit does

Measures airflow and temperature at that room’s vent against others, checks static pressure, and gets eyes on the duct serving it. That is usually enough to say whether it is a damper, a disconnected run, or the room simply having more load than the system was designed to give it.

Finding it is a check-up or a diagnostic depending on whether the rest of the system is behaving. What gets done about it depends on what it turns out to be, and anything needing new ductwork or a permit gets handed to a licensed Arizona contractor rather than attempted here.