Frost on the copper or across the indoor coil looks dramatic, and it worries people more than almost any other symptom. It is worth saying clearly: ice is never the illness. It is what the illness looks like from the outside.
Two very different faults produce identical frost. Starved airflow — a matted filter, a collapsed duct, a fan running slow — lets the coil drop below freezing because not enough warm room air is passing over it. Low refrigerant does the same thing for the opposite reason.
From across the room they are indistinguishable. Superheat and subcooling tell them apart immediately, which is why we take the readings before touching anything.
One thing you can safely do while you wait: switch the system to fan-only and let the ice melt. Running a frozen coil hard risks liquid refrigerant reaching the compressor, and that turns a small bill into a large one.
