The Evaporator Coils are Frosted Over

Hi @abhijitdesai,

An iced up evaporator unit could be due to a faulty defrost heater, a faulty bi metal defrost thermostat (aka thermodisc), a blocked drain running from below the evaporator unit to the evaporator pan near the compressor under the compartments or a faulty control board.

I can't find any info on the model number that you posted. Did you mean ZISS420DRESS?

If so here's the mini manual that may help as it has the wiring diagram for the refrigerator.

Disconnect the power to the refrigerator and then use an Ohmmeter to measure the resistance of the defrost heater. According to the manual it should measure 38 Ohms. If the heater is OK, here's a video that in principle shows how to test that the defrost thermostat is OK. If the thermostat is OK check that the drain tube is not blocked by pouring a measured amount of water i.e. 1 cup, down the drain and check that all of it reaches the evaporator pan under the refrigerator.

Here's a link to the parts diagrams that may help. If there is a faulty part, find it in the parts list and then search online using the manufacturer's part number for the part to find suppliers that suit you best.

If all the above check out OK it may be a control board issue. With most auto defrost refrigerators, they go into the defrost cycle once every 8-10 hours (depends on make and model). The defrost cycle lasts for approx. 20-25 minutes. During this time the compressor and evaporator fan are both turned off and the defrost heater is turned on to melt the accumulated ice from the evaporator unit. The meltwater drains down to the evaporator pan to evaporate naturally. During this time also, if the temperature near the defrost thermostat rises above its preset operating temperature - approx 46°F, it will operate and disconnect the heater to prevent damage occurring to the evap unit. At the end of the defrost cycle, the control board restarts the compressor and evaporator fan to drive the temps back down to their set operating temps. If the defrost thermostat has operated once the temp reach approx 26°F it will reset thereby allowing the defrost heater to work again for the next defrost cycle. Basically check that the defrost cycle is happening. This may be hard as I cannot find a method for this model how to manually start a defrost cycle so you may just have to be patient and catch it when it does.