An iced-up evaporator is a common service call for any service technician working in this industry. Generally, determining this is the cause of the system problem is rather easy. But what caused the evaporator to ice up may not always be obvious.
Frost or ice buildup is a normal part of any freezer operation, and is removed by using heated defrost cycles at timed intervals. During normal operation, the amount of this buildup should not get to the point that problems will occur. However, in some abnormal circumstances, it can lead to problems.