The drain pan.

The drain pan sits directly under the evaporator coil and catches the water that condenses out of your air all summer. From there it leaves through a PVC drain line.

In a Phoenix summer that is a great deal of water moving through a shallow plastic or metal tray, continuously, for months.

Often there are two

The primary pan is part of the unit, directly under the coil.

The secondary pan, or emergency pan, is a larger tray the whole unit sits in. It exists for attic installations, and its job is to catch anything the primary pan spills before it reaches the ceiling.

A secondary pan usually has its own drain line that terminates somewhere deliberately annoying, like above a window. Water coming out of there is a warning light, not a leak. Something upstream has already failed.

How a pan fails

It plugs, which is really the drain line plugging, and the pan is simply where the water shows.

It rusts through, on older metal pans. A pan that has carried water for fifteen years in a hot attic eventually gives up, usually in the corner you cannot see.

It cracks, on plastic pans, usually from being stepped on by somebody working up there.

It slopes wrong, after the unit shifts on its platform. Then water collects at the wrong end and never reaches the drain, and the pan looks full while the line looks clear.

Why it is the first place to look

A system that is completely dead with no other symptom is very often a float switch in the pan that has done its job. The thermostat still lights up, the breaker is on, nothing smells and nothing is iced.

Go and look at the pan before you look at anything else. It is free and it is right more often than any other single check.

Keeping it working

A cup of white vinegar down the condensate access tee every couple of months through cooling season. Flushing the drain is part of a tune-up, as is actually testing the float switch rather than just looking at it.