Freezer only getting down to 20 degrees, fridge side is fine. There was frost buildup on back interior wall of freezer. I removed the back interior cover to find ice buildup throughout the evaporator coil area and a big ice chunk around the copper tube coming into the evaporator coil. Thoroughly defrosted everything by hand. Checked the evaporator heater (good continuity). The evaporator fan is running fine. There is no defrost timer on this unit, it has an Adaptive Defrost Control (electronic board in fridge side control area). Put everything back together and fired it up. It got back down to 20 but no colder. I did the manual defrost override (pushed fridge side door switch 5 times in 6 seconds) which did start the defrost process. Everything defrosted, defrost heater got HOT (as evidenced by the blister on my finger) and it switched back into cooling mode. Now (four hours later) the temp has bottomed out at 20 again. I opened the back panel to find heavy frost on the top third of the evaporator coils and around the copper tube coming into the coil area as well as the area around the defrost thermostat. The bottom two thirds of the evaporator coils are free of any frost.

So, is this a compressor/refrigerant/sealed system issue or is there something I'm missing?

Thanks for the info. Do you have any idea how much such a repair might cost? I am wondering if it is worth the cost to repair or if we should replace the whole thing.

Also I have seen information posted elsewhere on the internet that says a partial frost pattern means a freon leak or a bad compressor, either way it is not cost effective to repair. I could replace this unit with a similar new one for about $800.