vokera linear 24/28 fault 04?

My mothers boiler was showing code 04 this morning
and the pressure was zero. I raised the pressure to 1.5bar as advised in the booklet, but when i tried to reset the system the dial/knob just turns all the way round so can't reset it. Called Engineer and he took one look and just said "oh, thats your circuit board gone", asked him if he was sure and he said i will get a new dial for tomorrow and we will see if it resets and that will save you a lot of money, if not you will need a new circuit board. Any advice please will be really appreciated.