Which of the following can cause an evaporator to freeze up?

An evaporator freezing up is often linked to low refrigerant levels. When the refrigerant is insufficient, it cannot absorb the heat effectively from the air passing over the evaporator coils. This results in the coil temperature dropping significantly, leading to condensation on the coils that can freeze and create a layer of ice. This ice buildup restricts air flow, further exacerbating the freezing issue and ultimately leading to inefficient operation of the system.

While dirty air filters can restrict airflow and contribute to freezing issues, the fundamental reason for the ice formation is typically linked to the refrigerant's ability to absorb heat. Low refrigerant levels directly impact the cooling cycle's effectiveness, making this the primary cause for the evaporator freezing up in this context.

In contrast, other options like by-pass type humidifiers and high ambient temperatures do not directly cause the refrigerant levels to drop. While they might influence overall system performance or encourage moisture levels, they do not create the same direct relationship to the freezing of the evaporator coils as low refrigerant levels do.