So I just build my first printer a Prusa i3 and it's been working well for a day.
I started a new print when suddenly it stopped printing. Smelled something burning and saw smoke so I quickly unplugged everything. After checking all the cables it seems to come from the heatbed.
Everytime I plug in the heatbed cables, my power cable would get hot and the bed warms up. The led behind D8 stays red when I plug in the power. Everything else still works. I can still control the hotend temp. Bottom of the heatbed doesn't show any burn marks.

Should I get a new RAMPS board or just change the mosfet? How do i check if my Arduino is affected?

Disconnect the wiring to the heatbed then measure the resistance at the heatbed terminals and see what you get. If you are using a RAMPS board, check the green power connector (that plugs into D8) for burn marks - that would indicate cables not being tightened sufficiently.

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Waitaki 3D Printer

After taking a closer look the mosfet to the heatbed was totally dark with burn marks. I just decided to buy a new ramps board.