Why does frost keep building up in my freezer?

Our sequence, and why it runs that way. Heater first: continuity, then a proper look at it, because one that has been failing on and off leaves scorch marks a meter will never report. Thermistor second, dropped into ice water against a reference, so we are testing the sensor instead of believing the display. Terminating switch third, since its whole job is to shut the cycle down and one that shuts it down early leaves half the coil still furred. Fourth the drain, because a defrost that runs perfectly and has nowhere to put the water simply lays it back down as ice. Last an independent probe on the evaporator, as the number on the front of the cabinet is under no obligation to match the metal behind it.

Now the door, and this one you can do before we arrive. Run the back of your hand slowly along the closed door seam, all the way round. Where the seal is doing its job you feel nothing at all. Where it has given up you feel a thin cold draft moving against your skin. Gaskets harden with age and lose magnetic pull, and freezer drawers in Ocean Beach kitchens see more traffic in one July than most units see in a year. A seal is an ordinary part. Left alone it turns into a defrost problem, and eventually into a compressor that never switches off.

If that matches what your appliance is doing, say so when you call — it tells us which parts to bring, and most of these are finished in one visit.