It never shuts off.
On a 115 day this may be the system doing its job. The rest of the year it is a question worth asking.
First, the uncomfortable truth about this city: on the worst afternoons, a correctly working system in a Phoenix house will run more or less continuously, and that is not a fault.
Most Valley equipment is sized to hold roughly 20 to 25 degrees below the outdoor temperature at peak. At 115 that is a house in the high eighties. The system is not failing, it is at the edge of what it was specified to do.
So the real question is not whether it runs constantly. It is whether the house gets where you set it.
The free checks
What is the setpoint, honestly? A thermostat set to 68 in July is asking for something no residential system in this city will deliver. It will run forever trying.
Is the fan set to ON? Then the blower never stops by design, and you may be hearing airflow rather than cooling.
Filter and vents. Starved airflow means less heat moved per minute, so the system runs longer for the same result.
The cheap mechanical causes
A dirty condenser coil. The one that matters most here. A coil that cannot reject heat stretches run times through the whole summer and shows up on the power bill long before it shows up as a breakdown.
A contactor welded shut. Then the outdoor unit physically cannot stop, even when the thermostat is satisfied. The tell is a unit that runs when the thermostat is off.
Leaky or uninsulated duct in a hot attic. You are cooling the attic.
The expensive ones
Low refrigerant. Reduced capacity means longer runs, and low means a leak.
A failing compressor that no longer pumps what it used to.
An undersized or badly matched system. Nothing is broken. It was specified wrong, and that is worth knowing before you spend money on parts.
The ones that are not the AC at all
A new west-facing window, attic insulation that has settled, a door seal gone, more people and more cooking in the house than last year. The system is being asked to do more than it was.
How to settle it
Measure rather than guess. Delta T across the coil, pressures, amp draws, run times, and the coil condition. Those numbers separate “at its limit on a brutal day” from “losing capacity”.
That is a check-up if it is running and you want a read, or a diagnostic if something is clearly wrong. Either way the numbers go in writing, and next summer the reading that moved is worth more than any single reading on its own.
Questions that come with this one.
Is it bad for the AC to run all day?
Long run times are easier on a compressor than frequent starts. What is expensive about constant running is the electricity, and what is worrying about it is if the house is not actually reaching setpoint while it happens.
How do I know if it is normal or not?
The question is not whether it runs constantly, it is whether the house gets to the temperature you set. A system running flat out and holding 78 on a 115 day is working. A system running flat out while the house climbs past 85 is not.
Would a bigger system fix it?
Usually not, and it often makes comfort worse. Oversized systems short cycle, never dehumidify, and are hard on the compressor. If the system genuinely cannot keep up, the answer is usually airflow, charge, coil condition or insulation, in that order of cost.
Other things that go wrong.
What the line items on an AC quote mean.
A translation, line by line, so you can tell what you are being asked to buy.
Read itRepair it, or replace it.
The questions that actually decide it, from someone with nothing to gain from either answer.
Read itIce on the AC line, in a Phoenix summer.
A frozen coil in July looks absurd and it is not. It means airflow or refrigerant, and running it that way risks the compressor.
Read it