I do residential, so I was just at a gas station I frequent. He asked me to look at his cooler. Complaint was freezing evap. He turned off the unit the night prior. Pan full of water, so I vacuumed out. Coil was icing up immediately. WR defrost with sencing bulb set at +30 whatever that means. Drain mustbe stopped up, but I suspect the defrost Iis not Iinitiating or not running long enough. But what do I know. R-502 frezone. I passed it to a buddy that can do low and med refrigeration. Know when to say nope.
Maybe a starved evaporator.
A coil that is filled with that greasy lint has decreased airflow, and the defrost doesn't complete itself. After the ice is gone, I usually find a layer of crud.
Time clock? If there isn't one, put one in. If the box is relying on off time from the stat then you can also get ice buildup over time due to inadequate defrost. Start with that first before as well as ensuring a full charge and the fact that the system does cycle off.