What is the most common problem with commercial washers in Arizona?

It is a failure of the inlet valve. An inlet valve turns the water supply on and off entering the washer. It is hooked to the inlet hoses which in turn are hooked to the valve on the wall.

In most of Arizona we have very hard water. Scale deposits from the water effect the rubber diaphragms inside of an inlet valve making them stiff or gummed up. Hot water valves fail more frequently.

Inlet valves are continuously operating anyway. The problem of hard water scale just makes them more susceptible to failure.

You will know that an inlet valve has failed when water continues to run through the machine even if it is off. The valve is stuck open. You can turn the water off at the wall.

Inlet valves are fixed by either replacing the rubber diaphragm or replacing the entire inlet valve assembly.