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if the red light on the pcb stays on after the tap is turned off this confirms the flow switch fault

have you cleared the air locks from the bleed points, these are sometimes at the top of vertical pipe runs open them with a screwdriver or bleed key. another thing you can try is turning all the upstairs rads off then all the pumps force is pushing to the rads that are air locked

Try removing the pcb and look at the back, top left corner of board for dry/fractured solder joints