Not knowing which might be bad I first replaced the Timer (easiest to replace). Achieving no appreciable difference after 24 hours I then replaced the Defrost Thermostat. This of course involved emptying the freezer into coolers to get to its location. After the freezer was emptied it was a simple job of removing two panels with a total of about 4 screws and several push-pin type retainers to get to the Thermostat. Simple job of cutting 2 wires and installing the new Thermostat with crush type wire connectors. Reinstall the panels and reload the freezer and all is well and freezing as is normal. Freezer's age 28 years bought new in 1993, Whirlpool.