montague

Hi, My Hot Tub is about 10 years old and recently started overheating. I set it at 35c and when it got to 35c it kept running and went to 36c and was still running and it switched itself to the fast pump level and eventually showed an OH sign (although it had not over heated) In all cases the water was not overheating as I checked it with a thermometer and the temp. was correct. I have removed the the filters and the gate valves are definitely open What would cause to keep running and the OH sign to come on when it has not overheated?? Any help would be greatly appreciated. Thanks Colin