vaillant ecomax 824e flashing e20,e13,f20. How to fix it?

Thing is, that my boiler (which is vaillant ecomax824e) use to go through these codes e20 and then f 20, or e13 and then f20. And if you resset, it was working as nothting happened. During last 6 month’s I noticed that :
• when I run the bath, I have to turn tap down low to get water hot, if I run tap at a higher flow it only gets luke warm. Same thing use to be before.
• while showering the water changes from hot to cold, when it suppose to be hot. And if you open hot water tap again, you‘ll get hot water,but after minute it comes cold again.
Lately this ‘water changing time‘ became shorter. And one day hot water disapeared at all, flashing on display screen e20 and was a little bit smelly like burned wire(but i wasn‘t sure of this smell, so haven‘t paid a lot of attention to this).After few ressets, it starts flashing e13 with some “bang bang“ in the boiler (when boiler starts to heat up water or if on central heating mode), than turns to f20. After night left swichted off and after some attempts to resset during next day , heating works fine and water we get, when heating on. One or two days passes like this, and we get hot water, from hot water mode. But same thing – at the begining hot, which in 30 seconds becomes cold. And two days hot water disapaered again. No, when I doing same thing with resseting, there is no hot water or heating also not comes on. Screen flashing e13 or f20, depends from temperature boiler reaches.When temperature 98 c it turns, on f20 mode.
Pressure in the boiler 1-1.5 bar, when temperature rises up to98 c and pressure comes out from right diapazone it turns, on f20 mode, and pressure comes back to right diapazone1-1.5 bar. On display everything works as normal. Boiler starts heating up (water or CH) it goes that selfchecking stages from 0 to 7 and then stops ,because starts e13 or f20 flashing. It selfchecking stages goes like that,when water tab open 0,1,2 then 6, when you close tab 7.And one more thing, when boiler starts work you can hear all these normal noises(when it‘s in normal work regime), except no “grr grr“ noise.
So after a lot of reading and comparing others problems with mine, I assume, it could be:
1. a diverter valve diaphram (or all diverter valve ),
2. also service kit in for the diverter valve
3. heat exchanger has blocked
4. pump(water pump, I suppose,which Carefree suggested, what‘s also makes sense now).
QUESTIONS :
• are my suspicions right?
• how to check is pump, diverter valve or heat exchanger working fine or it‘s time to change it?
• What are parts codes for diverter valve diaphram, diverter valve, diverter valve service kit, heat exchanger, pump(water pump). Should these parts be originals to vaillant ecomax824e, or only vaillant make parts, or from others producers parts also fits.
• Maybe ‘key‘ button could help the fault, but when I press and hold it for 15 seconds nothing hapens.What then is correct use of this button?


THANK‘S FOR YOUR PATIENCE