Kiln Controller

I wondered if anyone here has experience with kilns or thermocouples? I have a customer who teaches pottery and I’ve done several repairs on her hobby size kilns, usually just replacing elements. This time one of the wires to an element had burned through so I shortened the wire and fitted a new ceramic terminal block. There is still a fault though, with the computerised temperature display / control. It ramps up as normal but the ambient temperature is wrong, typically showing -25 C. Customer thinks it’s not firing at the correct temperature either, presumably due to the same fault that is affecting the start temperature. I wondered if it could be a thermocouple fault but the technical bod at the kiln manufacturer said thermocouples either work or don’t work. He said it must be the controller that needs calibrating. I removed the controller and it was sent away for calibration. They sent it back saying there is nothing wrong with it. 😕