In an effort to save money, manufactures and service companies scimp on a time clock and use off time cycling of the cooler box to control ice build up.
Any one who has been on call at anytime in there career will tell you it don't work well.
Everytime something goes wrong or someone leaves the cooler door open the evaporator coil ices up.
So before you can begin to properly diagnose the cause of the ice up, you first need to defrost the evaperator coil completely.
THEN, you can not leave the job until you witness the cooler cycle on and off on temperature and you verify it turns off at exactly 35-36 and back on at exactly 40-42.
Otherwise, you guessed it, you get to go back and check it out all out all over again in a week!