Since you don't identify one specific code and '1 ,2, 3, 4' are completely unrelated to each other, it sounds like the display and / or the display driver in the PCB is stuffed.
Have a look yourself at http://www.partsarena.com/baxi/ and see if you can relate the code(s) to the descriptions you'll find there.
envoys are pants
the condensate trap is always blocking and flooding the bleeding things, I always rip them out when I get one as you'll end rebuild it and STILL have loads of call backs
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