A frozen AC coil looks dramatic, ice on the indoor coil or the refrigerant lines, and diagnoses simply: only two things drive evaporator temperature below freezing, not enough warm air across the coil, or not enough refrigerant pressure inside it. Frozen AC coil causes therefore split cleanly into the half you can check and the half a gauge set must. This diagnostic covers both, plus the thaw.
Why coils freeze at all
The evaporator coil cools because refrigerant boiling inside it absorbs heat from house air blowing across it. The design balance keeps coil temperature above 32°F; upset the balance in either direction and moisture condensing on the coil freezes instead of dripping to the condensate drain, and ice then insulates the coil, which drops its temperature further, which makes more ice: the runaway that ends with a frozen block, weak warm airflow at the registers, and sometimes ice visibly climbing the large insulated line outside.
Starve the airflow and too little heat arrives to keep the boiling refrigerant warm: coil temperature falls below freezing. Starve the refrigerant (a leak, almost always, since systems are sealed and do not “use up” charge) and pressure in the coil drops, and boiling point falls with it, same result by the other door.
The thaw, before any diagnosis
Cooling off at the thermostat, fan set to ON: house air melts the ice over 2 to 6 hours while the compressor rests. Towels or a shallow pan around the air handler catch melt that overwhelms the drain pan. Do not chip ice off fins, and do not run cooling “to see if it happens again” before checking causes, because a compressor fed liquid refrigerant through a frozen coil is how a repair becomes a replacement.
The airflow half: yours to check
Run the list while it thaws. The filter, first and most likely, replaced if in doubt, and noting that an overly restrictive filter can starve a marginal blower even when “clean.” Supply registers and returns open and unblocked, furniture and rugs included. The evaporator coil’s own face: years of dust bypassing filters mat the fins into a blanket, visible with the access panel off, and a professional coil cleaning is the cure. The blower: a wheel caked in dust or a motor running weak moves less air than the coil was sized for. And duct restrictions, crushed flex duct, closed dampers, count too, cousin to the duct leakage story. If the freeze followed a mild night or a low thermostat setpoint, note it: low load plus marginal airflow is the classic combination, and “it only freezes at night” is airflow’s confession more often than refrigerant’s.
The refrigerant half: the technician’s
Airflow verified clean and the ice returns anyway: the remaining cause is low charge, and low charge means a leak, since charge does not deplete in a sealed system. The visit worth paying for finds and fixes the leak (coils, joints, valve cores), then weighs in charge to specification; the visit worth skipping “tops off” refrigerant annually, which is renting the same failure by the season while the leak grows. Ice specifically at the outdoor unit’s small line and valve in cool weather, or a system that freezes in mild conditions with perfect airflow, strengthens the refrigerant case. Heat pumps complicate the picture only slightly: outdoor-coil frost in winter is normal and defrost-managed, per heat pumps in cold climates; indoor-coil ice in cooling season follows this article’s rules.
Preventing the rerun
Filter on schedule, coil cleaned when the panel-off inspection says so, returns kept free, condensate path clear, and one professional maintenance visit that includes coil condition and a refrigerant check on the multi-year rhythm. Frozen coils are almost never the first event in their story; they are the loud chapter after quiet months of restriction or a slow leak.
The bottom line
Ice means starved airflow or starved refrigerant, nothing else. Thaw with the fan, check the filter-registers-coil-blower chain yourself, and hand the recurring or airflow-innocent cases to a gauge set with instructions to find the leak, not feed it.