Why is my MacBook Pro's fan constantly on at 6000RPM?
This is a guess but I would assume it's referring to the Temperature sensor next to the heat sink exhaust for the right fan.  That would definitely account for your fan running at max... if the sensor is not reporting the temperature, the logic board would likely turn the fan on full blast in order to keep the processor from overheating or even setting the computer aflame.

You might have just forgot to plug the temp sensor in, or left it loose, or it became loose during your board replacement.  Or it could have failed...

Try using a temperature monitoring app like Temperature Monitor and see if it gives you any more info.