Get the AC ready before the first 110 day.

The useful thing about a Phoenix summer is that it is predictable. The first 100 degree day usually lands in April. By June the system is running most of the day and much of the night, and it does not really stop until September.

That means March is the cheap month. Anything found in March is an appointment. The same thing found in July is an emergency, on a day when every company in the Valley is already booked.

Twenty minutes, yourself, for nothing

Change the filter. Not inspect it, change it. If you cannot see light through it, it has been restricting the system for a while already.

Open every supply vent, including the rooms nobody uses. Closing vents does not save money. It starves airflow across the indoor coil, which costs capacity and is one of the two things that freezes a coil.

Pull the furniture off the return grille. Same reason, bigger effect.

Walk out to the condenser and look at the fins. A winter of dust and a spring of blown debris sits in that fin pack. If you can see a grey mat through it, a rinse is worth doing and a chemical clean is worth considering.

Clear two feet around the outdoor unit. Oleander that grew into it over the winter, bags leaned against it, a trellis somebody added. The unit rejects heat sideways and upward, and it cannot do that into a hedge.

Pour a cup of white vinegar down the condensate access tee. The line has been dry for months and it is about to be wet for five. This is the cheapest thing on the list and it prevents the most annoying failure on it.

Run the cooling for fifteen minutes on a mild day. Not as a test of capacity, which a mild day cannot measure, but to find out whether it starts, whether the outdoor fan spins, and whether anything sounds wrong. Finding out in March is the entire point.

What is worth paying for, and when

If the system is running and you want to know where it stands, that is a check-up. Measurement only: pressures, amp draws, capacitor microfarads against the rating on the can, temperature split, airflow, and photos of the coils. You get the numbers in writing and you decide what to do with them.

If you want the maintenance done as well as measured, that is a tune-up. Everything in the check-up, plus the coil rinsed, the drain cleared and flushed, the float switch actually tested rather than looked at, and the electrical connections checked and tightened. There is a full list of what that covers.

If the outdoor coil has not had a proper clean in several seasons, the deep clean tune-up is the one that matters here. Monsoon dust packs into the fin pack and sets, and a garden hose moves the surface of it and nothing else. A coil that cannot reject heat makes the compressor work harder all summer, which shows up on the power bill long before it shows up as a breakdown.

The part most likely to quit

The run capacitor, and it is not close. It is a heat-aged component living in the hottest place available, and it usually fails on the first properly hot afternoon rather than declining gently. Measuring it in March takes a minute and tells you whether it is already out of tolerance.

A capacitor replaced in March is a scheduled visit at a published price. The same capacitor in July is a house at 90 degrees while you wait for a call back.

The honest version

Plenty of systems need nothing. If yours measures well, that is what you will be told, and you will have paid for a check-up and nothing else. The written record is still worth having, because next March you will have something to compare against, and a reading that moved is worth far more than a reading on its own.

What is not worth doing is finding out in July.