I had the same problem with my Whirlpool fridge. I replaced the thermistor which snaps onto the heating element and plugs into a plastic connector. The repair was easy except for the removal of the inside panel at the back of the lower freezer drawer. The drawers need to be removed first. It is easy to figure that out too but be gentle as these plastic parts are brittle and can break easily. these aren't cheap to replace so be careful. Go to PartsSelect.com for diagrams and replacement parts. The part was $47 and I saved $300-$400 in repair costs by doing it myself. The only tools I needed was a Philips screwdriver.