What causes an ac evaporator to ice over?

An AC evaporator can ice over due to a few key factors, primarily related to low airflow or insufficient refrigerant. When airflow is restricted—often due to dirty filters, blocked ducts, or a malfunctioning blower motor—the evaporator coil can become too cold, causing moisture in the air to freeze on its surface. Additionally, low refrigerant levels can lead to lower pressure in the evaporator, resulting in temperatures that drop below freezing. Both situations can disrupt the normal heat exchange process, leading to ice buildup.