Ductwork.
The part of the system nobody looks at, running through the hottest place in the house.
Ductwork is how conditioned air gets from the air handler to the rooms and back again. It is half the system and it gets a fraction of the attention, largely because most of it is in an attic nobody visits.
In Phoenix that attic reaches well past 150 degrees on a summer afternoon, which makes everything about the duct matter more here than it does elsewhere.
What is in a Valley house
Flex duct, which is a wire helix with an inner liner and insulation wrapped around it. Cheap, fast to install, and easily crushed, kinked or pulled off a fitting.
Rigid metal, usually on trunk lines, sometimes wrapped, sometimes lined.
Ductboard, a rigid fibreglass board formed into rectangular duct. Common in older installations.
Why leaks cost so much here
A duct leak in a 150 degree attic is not a small inefficiency. Supply leaks dump air you already paid to cool into the attic. Return leaks pull 150 degree attic air into the system, which the equipment then has to cool from there.
That shows up as a system running constantly, a house that never quite gets comfortable, and a summer bill that makes no sense. Nothing has failed, so nobody looks.
The common faults
Disconnected runs, where flex has pulled off a boot. A room that went from fine to terrible suddenly is the classic sign.
Crushed or kinked flex, from somebody standing on it or storage piled on it.
Insulation gone or compressed, which turns the duct into a radiator.
Undersized returns, which is a design problem rather than a fault and shows up as weak airflow everywhere at once.
Leaky connections at plenums and boots, where the tape gave up twenty years ago.
What can be done here
Finding it is straightforward: airflow and temperature at the registers, static pressure, and eyes on the duct in the attic.
Fixing it depends on what it is. New ductwork and duct replacement are off the list here, along with anything needing a permit, and those go to a licensed Arizona contractor. You will get told plainly which side of that line your problem falls on, which is more than a quote usually explains.