Thanks, i sealed the huge crack from the MAF/filter and code has yet to come back, but still a little power loss... seems like my TCV is bad again since it will go then boost will drop, then come back, so much for that IPD one i bought. :/the system assumes that all the air that passes through the MAf ( assuming that's working properly) reaches the cylinders. The engine management system sets the air/fuel ratio dependant on the amount of air and lots of other factors) load, air temp etc) and the feedback from the O2s.
If air is sucked in after the MAf but before the turbos or blown out after the turbos ( slightly higher pressure than atmospheric) the ratio will be off and picked up by the O2's
That then causes the system to compensate via fuel trim ( + or -). When it goes out of a predetermined range it sets the fault codes and changes to prefixed "substitute " values
It all has to be airtight.