Frozen AC unit or evaporator coil: thaw it, then find the cause

The evaporator coil normally runs cold enough to pull moisture out of the air and stay just above freezing. Two things push it below that point. If too little warm air passes over the coil, the refrigerant cannot pick up enough heat. If the refrigerant charge is low, pressure in the coil drops, and its temperature drops with it. In both cases, condensate freezes on the fins, the ice blocks more air, and the problem feeds itself.

You may see ice on the indoor coil, on the larger insulated copper line running to the outdoor unit, or on the outdoor unit's service valves. Indoors, the signs are weak airflow, air that is no longer cold, and water around the unit when the ice melts.

A full thaw takes time. A lightly frosted coil can clear in an hour or two. A coil encased in ice can take most of a day. Starting the AC before the ice is gone just refreezes it.

1. Dirty air filter. The most common cause and the least expensive fix.
2. Closed or blocked vents. Too many closed registers or a blocked return starves the coil of air.
3. Low refrigerant from a leak. Often shows as ice starting at the coil's inlet and on the copper line. See the guide to an AC refrigerant leak.
4. Dirty evaporator coil. Dust that got past the filter mats the fins and blocks airflow from the inside.
5. Weak blower. A failing blower motor, a bad blower capacitor or a dirty blower wheel moves too little air.
6. Running the AC in cool weather. A standard AC is not built to cool when it is chilly outside, and the coil can ice up on cool nights.
7. A stuck contactor or thermostat fault. The outdoor unit keeps running after the indoor fan stops, and the coil freezes with no air crossing it.

The coil has to be thawed before anyone can diagnose it, because gauges on a frozen system give meaningless readings. If you thaw it before the visit, you save time and money.

The technician then checks airflow: filter, blower speed, blower wheel, coil surface and ducts. Next come refrigerant pressures and line temperatures. If the charge is low, the right step is a leak search. Evaporator coils are a common leak location, and the evaporator coil replacement cost guide explains what that repair involves.

You can fix the filter and the vents. You cannot check refrigerant charge or the inside of the coil yourself. If the system freezes a second time after you have done the basics, stop running it and book a visit.

Airflow causes are maintenance, and they do not call for new equipment. A leaking evaporator coil is the case that raises the question. If the system is old, out of warranty, or uses a refrigerant that is no longer produced, the price of a new coil plus refrigerant can be a large part of the price of a new system. Running a system that keeps freezing also risks the compressor, because liquid refrigerant can flow back to it. The AC repair overview covers how to compare the two paths.