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Hi if the boiler has the 4 led's on the front, then its most likely going to be one of the thermistors in the boiler,
if the boiler is overheating just on water only then try replacing the one in the slave tank on the side, if its water and heating then the one in the main heat exchanger. if you lift the lid on the boiler you can see the white wires going to the relevant pockets.