This could just be a 3 or 4 wire type thermocouple. 3 or 4 wire are supposed to be more accurate because they are able to compensate for the resistance through the actual wires themselves and not just the resistance across the thermocouple. If it is a 3 wire thermocouple that came with your system, your controller (depending on the brand) should be able to handle the change from a 2 to a 3 wire. You might have to re-run the wires that go from the controller to the thermocouple to change it from 2 to 3 wire but then it should hopefully work. That's about the extent of my thermocouple knowledge but hope it helps some.