This thread is to try and consolidate everyone's experience with this very annoying CEL. Please post what you have tried and what has solved the problem, or what your dealership has done to resolve this for you.

I have a 2008 R55 Clubman S, after 40,000kms this 2786 CEL comes up within 50kms-100kms after clearing it, sooner with town driving. Currently have 46500kms, last 6500kms have had this issue, it is getting worse.

Mods:
Alta CAI was put on the car at 750kms, so it has been on a long time. At this time the boost tubes were also changed (Alta too), so before and after turbo. The tube going to the intake manifold failed and also caused endless rubbing issues, a terrible hose to say the least. I put back the stock hose there.

Tried so far:
Replaced valve cover, did not help.

Notes:
Issue seems to always occur in 1st gear on take off. Half power/turbo boost cut above 4000RPM. Eventually Engine Check light comes on, but never right away, can go on for days before light comes on. It has corrected itself too, rare, but as long as the light was not on, the power has come back until it happens again.