What causes AC to freeze up?

An air conditioning system freezes up primarily due to disruptions in the delicate balance between airflow and refrigerant temperature within the closed-loop system. The most frequent culprit is restricted airflow, which stems from severely clogged air filters, dirty blower wheels, closed return grilles, or crushed ductwork that starves the indoor evaporator coil of the warm air it requires to stay above freezing. Another major cause is a low refrigerant charge resulting from a puncture or loose service valve, which creates a severe pressure drop and causes the boiling point of the remaining refrigerant to plummet well below the freezing mark of water. Mechanical malfunctions, such as a failing blower motor or a weak capacitor causing the fan to spin too slowly, also prevent adequate air movement across the coil. Additionally, a heavy accumulation of dust and grime on the evaporator fins acts as an insulating barrier, blocking efficient heat absorption. Finally, running your air conditioning system during unusually cool outdoor weather, such as chilly spring or autumn nights, can cause system pressures to drop and trigger unintended freeze cycles.