Ferolli optimax He31 c d1 fault code?

Hi there. I am showing d1 on the display of my ferolli optiamax 31c he boiler. I have hot water (which is now very hot) and no central heating.

Boiler is 5 years old and out of warranty. Scouring the web has indicated that it may be a faulty diverter valve.

Please can somebody highlight this as it does not feature in the operating manual provided by ferolli.

We have had to bleed the rads recently and this caused pressure to drop off. Refilled the system with the two black valves underneath and brought pressure back up to 1.5 bar.

Since then we have had hot water and d1 fault but with no central heating.

Any help would be greatly appreciated.