How to fix cooling fan on Jenn-Air JGS8850ADB to cool upper range area?

The fan is not coming on at a fixed temperature to cool the upper range area of the oven. The F3 Fault Code came up after oven was used. The Heat Sensor Thermostat was replaced just one week ago. However, the fan should come on at a fixed temperature approx 350-375 degrees. The appliance repairman said he thought fan was good and likely a clock/control board replacement. What is your opinion? Also, repairman said that the nut holding down burners break the tubes when trying to unloosen them because they become almost welded together, what is solution?