CBV is the compressor bypass valve. Basically it prevents excess crankase pressure from building up, and provides an out to circle air back into the system on the other side of the intercooler (I think... can never remember correctly).

Anyway, if you have a garret turbo then the CBV is a bell looking thing located directly on the front left end of the engine. If you have a mitsubishi turbo it's located on the turbo body itself. You can find it by tracing the small vacuum hose that goes from the intake manifold over the crankacase.

The garrett version is held against a plate with some allen screws, and if they get loose it can allow air to leak out and affect the mixture, etc.

BTW, I failed to say thet your mechanic is totally correct. Knock sensors AND ECT sensors don't usually fail. They CAN fail, but they are both tougher than the experiences of folks on this board would lead you to believe. The AMM and o2 sensor are a little more prone to failure, probably because of their more exacting operating conditions.

good luck,
rt