CONCERN: Customer reports SUB ZERO freezer is building frost and ice. DIAGNOSTIC AND CAUSE: Inspected compressor operation, evaporator fan and condenser fan operation, all normal. Found frost build up on evaporator coils. Tested defrost heater and thermostat for continuity, normal. Found drain trough heater shorted. CORRECTION: Defrosted extreme amount of ice and frost from evaporator. Replaced drain trough heater.. Tested operation, all normal upon completion.