Why does frost keep building up in my freezer?

What is actually going on

Three parts account for nearly all of it: a defrost heater that has burned open, a thermistor that has drifted far enough out of range that the control no longer triggers the cycle correctly, or a door gasket whose magnet has weakened, letting more moisture into the cabinet each day than the cycle can carry back out. The reason to deal with it early is that ice on the evaporator acts as insulation, so the unit does not fail, it fades. Cooling gets a little worse each week and the number on the display stops matching the temperature around the food.

What we check, and in what order

Coastal San Diego brings this fault out earlier than a dry climate would. Along the water the marine layer keeps indoor humidity high for most of the year, and every door opening loads the evaporator with more moisture than the identical freezer would see inland. A defrost circuit that is only marginally weak survives inland and gives itself away on the coast. It is also why gasket condition matters more here than the price of a gasket suggests.

There is a test you can run yourself in ten seconds. Close the freezer door on a strip of paper and pull it out. If it slides free with no drag, the seal is not making contact at that point, and the worst spot is usually on the hinge side or at a lower corner where the gasket has taken a set. On built-ins carrying a heavy custom panel the door hardware can also drift out of adjustment, which loads one corner of the seal and unloads the opposite one.

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