Why does frost keep building up in my freezer?

The reasoning behind that

Three faults cover nearly every case: a defrost heater that has burned out, a thermistor drifted far enough out of range that it no longer triggers the cycle, or a door gasket letting humid air in faster than the cycle can drive it back out. On built-ins in Mission Hills the gasket is worth checking first. The custom wood panels on these doors are heavy, hinges settle over the years, and a door that no longer pulls itself square lets damp morning air into the cabinet every hour of the day.

How the diagnosis goes

Test the gasket with a dollar bill before you call. Close the door on it at several points around the frame, top, bottom, hinge side and handle side, then pull. It should drag everywhere. If it slides out easily anywhere, and especially near the top of the hinge side where a heavy panel does its damage, the seal is the first thing to deal with and it may well be the only thing.

The moisture is not exotic here. Mission Hills gets the marine layer most mornings for a good part of the year, and these houses are not sealed the way new construction is, so the air going into the cabinet carries more water than the same appliance would see inland. A gasket in good order and a defrost cycle that finishes will handle that. A gasket that has hardened will not, and the ice builds on the coil where you cannot see it long before it shows on the walls where you can.

If that matches what yours is doing, say so when you call — it decides which parts come on the van.