FFU11FK0DW2 Frigidaire Commercial Upright Freezer - Not cold enough

[electropower]

This freezer is 20 years old, and has worked well. Today, my wife noticed that the items on the upper shelves were thawed. What would be the most likely root cause for this model?

Also, is there a factory repair manual available?

Thank you.

[electropower]

I removed the rear interior panel to expose the coils, and the coils had no ice. But there was ice at the bottom of the freeze. Does this indicate a blocked drain?

[electropower]

I melted all the ice, sent a rubber hose down the drain to try to dislodge any clogs, then wrapped a piece of copper wire around the heating element and put it into the drain hole. I put everything back together and turned the freezer on. Not sure if this will solve the problem.