Freezer section works, refrigerator becomes warm.

Question

I open the back wall in the freezer and clean all the ice on and around the heater which makes everything work fine. But after 3 or 4 days, the problem comes back and I have to do the same thing. The fan works fine. I am thinking that the heater does not defrost, that's why the ice accumulates. Need help, what part to change.

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