The evaporator coil is the indoor component responsible for actually absorbing heat from the air passing over it, which is the core of how an air conditioner cools your home. When something disrupts the delicate balance of airflow and refrigerant temperature around that coil, ice can form on it, which sounds harmless but actually makes the whole system work worse, not better.
Restricted airflow is the most common cause
A dirty air filter, blocked return vents, or closed supply vents throughout the house all restrict the amount of warm air passing over the coil. Without enough warm air moving across it, the coil's surface temperature can drop below freezing, and moisture in the air condenses and freezes directly onto it.
Low refrigerant from a leak
A refrigerant leak drops the pressure inside the system, which can cause the coil to run colder than it's designed to. This is a common cause of a frozen coil in a system that's otherwise been running fine for years, since it usually develops gradually as a small leak slowly reduces the charge.
Running the AC in cooler temperatures than it's suited for
AC systems are designed to operate within a certain outdoor temperature range. Running a system on a mild day when it's not really needed, especially with the thermostat set unusually low, can sometimes contribute to coil freezing, since the system isn't seeing the typical load it's designed around.
A dirty coil itself
Dust and debris that accumulate directly on the coil's surface over time act as insulation, disrupting normal heat exchange in a way that can also contribute to freezing, separate from an airflow or refrigerant issue elsewhere in the system.
What to do if you notice ice on your system
Turn the system off and let it thaw completely, running the fan-only setting can help speed this up, rather than continuing to run it in cooling mode. Running a system with active ice on the coil can push liquid refrigerant back to the compressor, which risks damaging it, turning a simple airflow fix into a much more expensive repair.
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