OK so it looks to me like a Mightyboard or a copy of it. The head temperature sensors are thermocouples, you can clearly see the amplifiers at the top left of your picture. In this case you need to measure the voltage they are putting out on their own without being connected to anything.

The full schematics are available so you can easily check what the problem with your board is. I am sure the problem will have been seen before so Google will find a solution I am sure.