I seem to have exactly the same problem, except that my very expensive report from the dealer said that code P1518 was the IMRC stuck closed! I've had a poke about and found the IMRC to be working fine - as you described; I can only imagine as you said, that the unit is reporting a non-existent fault. Your garage were pretty cheap - mine tried to fleece me out of £300 for replacing a unit that doesn't appear to need replacing! If only someone could help us diagnose the problem! If I find anything out, I'll report back!