GE Refrigerator Heater not working

Greetings,
My refrigerator makes a lot of noise with the fan sounding like it is picking up speed, then slowing for hours at a time. I replaced the fan motor but no help. Yesterday, I took off the inside back panel of the refrigerator and found most of the coils covered with ice. I noticed that the heater at the bottom of the coils was not hot. I disconnected the heater and it has continuity. It measured about 28 ohms which looks like it is OK. Any suggestions as to what I can do to make the heater do its job and defrost the coils.

Thanks for any help you can give.

John