i had same problem with my 2003 with a beowulf fitted,
the guys at the dealers workshop said it doesnt need it with having that can on and said they can put a eliminator to stop the tach error,but didnt make any difference, but when the main mechanic came back from being off ill, he rang me and said it needs the exup sorting its not right without it,he had it for a week, he put new cables and valve in, as he said the one in was completely siezed, and now its been done what a difference it makes,I noticed the difference in sound soon as he started it up and now runs much smoother at low and high speed.