Thetford truma ultra heat heater

Hi there 

I have a thetford truma ultra heat heater and on 240v it does nothing but blow out cold air. I have had my pcb away for repair and the guy tested it and there was no fault found and it was returned. When I have put it back in the heater it was the same fault. I have taken the front off the fire and the element it got hot to touch when it was set to 500 but when set to 1000 or 2000 the element cooled down to cold. 
can anyone shed any light on what the fault could be please? Thank you.