it won`t be the dhw flow restrictor bingo, its tripping on ch as well
although if op is draining down boiler to replace thermistors ( pn8484 )
i would chuck it out at the same time
Unfortunately these are particularly difficult boilers to diagnose intermittent faults on.
Of course there are several mostly gas related settings which an engineer would check and any of these might be causing your problem.
The trick is to eliminate those which you can check and then wonder what is left which could be causing the problem.
That usually leaves the PCBs which are both unreliable and expensive.
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