I have had a similar problem with bed going to max setting even though printer said bed temp was zero. Problem, I suspect, is that they do not put arc suppression on relay contacts so they are when open or close which damages contacts and eventually welds them shut. The fix is to take a 47 to 100 ohm resister and a 1 micro farad capacitor, solder them together in series that is one end of the resistor to end end of the capacitor, do not solder the other ends together, now take the ends of this device you have just made and solder one end to one contact of the relay for the bed, and solder the other end to the other contact of the bed, you can also do this by inserting the ends across the two screw lugs with the wires that are there. This will suppress the arcing in the relay and protect the contacts. I haven't had a problem since.