Why is my MacBook Pro's fan constantly on at 6000RPM? I recently replaced the logic board on my Mac, now I have this "fan constantly on at 6000RPM all the time" issue.

So I did an Apple hardware test using the installation disk that came with my Mac and got this cryptic error:
4SNS/1/40000000:Th2H-128.000

After some googleing I came across this:

Th2H Right Fin Stack Proximity Temp

And my question(s), what part does the Right Fin Stack Proximity Temp refer to?

Is the error given by the Apple Hardware Test considered 100% accurately?
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.