Freezer too warm

My GE side by side m# GSL25JFAPBS is too warm in freezer compartment, around 32 F.
I took the back cover off and found the lower 3rd of the evaporator coils frosted over and the upper portion with no frost (see pic). I ohmed the defrost heater and found 21 ohms, assume it is good. I checked the defrost thermostat and it stayed closed during testing in a cup of ice and blowing a hair dryer over it out of circuit, so I assumed it is bad. The evaporator fan is blowing well. I got a new thermostat today and installed it but after about 3hrs the symptoms are the same. Could use some advise on what to do next.

Thanks

After running empty for 24 hrs the temp is down to 19F in freezer but I don’t think this is low enough. Anyone have any ideas?

Here your parts. I think the "AP" in the model number should be PA.
http://www.appliancepartspros.com/partsearch/modelsearch.aspx?model=GSL25JFAPBS

Looks like you may have a low freon problem.
http://www.applianceaid.com/frig_notcold.html
Take a look at the example pictures in the 5th paragraph.

I would check that the condenser/compressor fan is running and that the condenser coils are clean.

If low freon is the case then the repair will be expensive as you will need a pro.
Finding and repairing a leak can be time consuming. Also there is no guarantee of a long term fix.
In most cases it makes more sense to put those dollars towards a new unit.

Check the purchase date as the sealed system should have a 5 year warranty,