The problem might be from a temperature sensor or a thermostat 温控器. the temp sensor might be installed under the floor tile. Try to replace 温控器 first. You should have more than one thermostat in your system. Turn off power supply of the redaint. Disconnect and replace a good 温控器 on the problem loop. You have to ask a handyman do it if you are not familar with electicic work. The safety is the first.
What kind of 师傅 is he? A good 师傅 can fix the problem within a day.
Good luck.