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Jw89
If the thermostat does not open it will not circulate the hot water to the radiator to circulate it for cooling, ergo the fan switch will not get hot water to it to cut the fan in.
A lot of modern engines will over heat with out the rad cap in as the pressure build up raises the boiling point of the coolant, without the pressure small pockets of coolant in the hottest part of the engine can boil up creating steam which then forces the coolant out.
I would replace the thermostat first.
checking the radiator is quite easy remove the hoses and force water through it to see if it flows OK