Appliance type: Top freezer refrigerator. Mechanical controls; thermostat located in fridge section.

The temperature of the freezer is 18-20F rather than the usual 0F - ice maker not working (presumably it only runs below a certain temperature) and some things are still soft.

The fridge section is fine and the compressor cycles on and off normally. The gaskets look okay too.

Could an iced up evaporator cause that? I suspect that it's not defrosting.

 

I had an old GE top freezer refrigerator (new in 1970 or 1971) at another location that was a little like what you describe, the ice maker hadn't worked correctly for many years. I found that the condensing coils under the refrigerator were totally packed with dirt and had probably never been cleaned in at least the last 25 years and maybe never had been cleaned at all. I blew them out. After the cloud settled the biggest problem was cleaning up all the dust that settled in 3 rooms. The old GE started working like a champ again. The old ice maker even started working again. If the refrig were not going through the defrost cycle, the coils would ice to the point that both sections would be warm and ice would have started appearing inside the freezer section. I had a very old bottom freezer Frigidaire do that once. The solution to that was about once a month to unplug the unit overnight and empty the condensate drain pan the next morning. It would work like new again for another month when ice would start appearing inside the freezer compartment again. It probably just needed a new defrost timer, but I was really dumb and poor back then.

 

There is the obvious stuff like making sure nobody (children) played with the controls. Many units have a fridge/freezer balance knob in addition to the temperature control which adjusts how much cold is sent to the fridge versus the freezer.

You provided the temperature for your freezer. Did you also check the temp of the fridge to confirm that it is not also running warmer than usual? If so it is possible that the defrost system is not working properly allowing the coil to ice up. Another possibility is simply old age or loss of refrigerant or a dying compressor.

If you find that it's not defrosting properly you should look up your model online to learn the layout in the coil area. There should be a heater under the coil and one or two temperature sensors. Any of which are possible failure points and are usually relatively inexpensive if you order the parts online. The killer is that you have to remove everything from the freezer to be able to remove the panel and access the area.

 

The vents in the freezer were partially blocked.

It's cooling better now - food is hard again but ice-maker still not working.

I'll keep an eye on it - thanks for the suggestions.