I have a Samsung RF261BEAEBC/AA Frend Door refrigerator, purchased in 2013. About 1 year after purchase there was a noise coming from the inside which I have traced down to ice build in the top right corner, and evaporator fan banging against this ice. Since then every few months I need to do a full defrost (shut down refrigerator and let its sit for 24 hours) to melt the ice. Ice only builds up along the top right side of the back panel. With the back panel open - and after all the ice has melted, I forced a manual defrost of the refrigerator by pressing power freeze and power cool simultaneously and then selecting "rd" mode. I can feel that the defrost heater is getting hot, but it only stays hot for about 5 minutes. I am not sure if this is normal (defrost heater switching off in 5 minutes) because there is no ice, or if this is not normal and the defrost heater is switching off too quickly.

Do I have a bad sensor, a bad heater, a bad board, or something else?

Thanks
Aspi