Hello,

I have frost forming on the back wall of my sub zero 511. The freezer is working fine. I've read that the defroster timer could be bad so I plan to replace that as the part is inexpensive.

I am wondering if the refrigerator evaporator coil is also iced up. Should I look to see if ice has formed there? Would this tell me anything new. How can I remove the back panel to view the coil?

Before you go to replacing parts helter skelter you need to test things out with a meter. You cannot just change parts and fix this type refrigerator as it's more like a commercial unit. I doubt the defrost timer would go out before the defrost heater. It has a commercial timer that lasts a very long time, but it could be bad. You can replace this timer with a universal type timer by Ranco or another manufacturer if it is bad. Look on page 4-12 for the parts list and 4-13 for the diagram. The defrost heater and terminator are on the coils and the timer is at the bottom. Make very sure you clean the coils well with spray soap and use a hose nozzle to wash them out. Clean the fan blades, too, as this is a very important step. Just a little bit of dirt on the fan blades will cut the airflow by 30%. The defrost heater, if it's a 100 watt heater should have about 144 ohms resistance. If it is bad replace the terminator along with it. If the Sub-Zero model is too expensive you can get generic ones very cheap. The same holds true for some defrost heaters if you match the wattage. I have used flexible heaters that work very well. Here are some manuals: https://dl.dropboxusercontent.com/u/8010163/Sub-Zero_500_series_refrigerator_b.pdf
https://dl.dropboxusercontent.com/u/8010163/How%20to%20use%20Test%20Equipment%20Service%20Manu al.pdf

Thanks for the reply. I have a question:
As there are 3 fans on this unit, which fan are you referencing, the unit condenser fan or the refrigerator evaporator fan or both? (I am assuming not the freezer fan)

Any fan that is dirty will not move enough air. When I said to clean the fan with the condenser I was referring to the condenser fan under the unit. If the freezer fan is not working right from lack of lubrication or dirt it will cause a lack of cooling, too. It's important with this unit to clean the refrigerator coils and the fan, too, because it gets very dirty. (This is true of commercial coolers, too.) A good foaming cleaner like Easy-Off Oven Cleaner (The kind with lye in it where you need rubber gloves.) works well on any aluminum evaporator coils in a refrigerator section or an evaporator on an air conditioner if you do not have any alkaline coil cleaner. Make sure to oil the fan motors in a Sub-Zero refrigerator. I have had to punch a hole in aluminum cover over felt oil reservoir to oil it.