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Hi C20 What please system or combi?
I presume you are saying Thermostat overheat had tripped so
If it has isolating valves inline make sure they are open, make sure the pump is working unscrew the big screw in the middle to bleed it out.
its probably not resetting as its was too hot.
Take the pressure down to 1 bar when cold please. 3 bar is way over pressure and will open the PRV when it gets hot.