Hello everyone, I have a Samsung side by side(RS21DASW) and it's not working properly. First of all, last year the board was changed, so it's a new one. The problem is the defrost system, when I turn on the refrigerator and run the self-diagnostics it doesn't show any error. But, after it runs the defrost cycle it shows an error (number 7 and 13 leds on, from the manual) R-DEFROST ERROR, F-DEFROST ERROR: In the refrigerator room, if frost removal mode is finished due to limited time of 80 minutes. Error is displayed.
There's accumulated ice on both sides.
I tested the sensors manually with a multimeter and everything was fine(as the self-diagnostics showed) according to the manual and the heaters for continuity. I'd like to know how I can discover what's wrong. If it's not the sensors, then it's the heater?
I tried to run a forced defrost, but my fridge was beeping a lot, then I gave up. How can I run a forced defrost?
If it's not clear, please ask me, I really need some help here.

Thank you in advance!
Mariana