check out5 my post 21 here :-
https://www.volvoforums.org.uk/showt...nnected&page=3
Gives a bit more info re dem
Dem & pressure switch can't be tested by "us" Thats why I suggested Lucus. As you say DEM is easy to remove ( if x2 bolts are not seized solid - metal bolts into aluminium casting) and send it off to him If its fault free IIRC cost is under £50. which you can soon spend chasing your type of fault.
Cheers
Bob