return air grill. Sometimes people don't understand that there should only be one set of filters on an HVAC system. If a previous renter noticed no filter in the return air and added one (since there was already one inside the furnace), you may still have a clog in the system. One or the other, not both. Return air grill is usually preferred since it's easier to get to.

Check, I've seen it more than a few times in homes I've had inspected.

Google says there are other reasons for A/C systems to freeze.