Definitely not the O2 sensors. They rarely fail and impossible they both fail. And if they fail, they report a rich condition because they generate no voltage as opposed to 1.5V (indicating excessive O2 in the exhaust, symptomatic of a lean condition)
If you have a lean condition on both banks you have a vacuum leak (that is unmetered air into the intake) It's one of two things: either the rubber intake boot where it branches of to the idle control valve or the crankcase ventilation system.
Since the diagnosis was wrong, I have no confidence that they did the smoke check accurately so I would still check the intake boot. Diagnosing the CCV it's a bit more complicated, but if you pull the oil cap and feel a strong vacuum than it's gone, although the plumbing cracks as well so the valve itself could be working, but still sucking in air from elsewhere. In my case neither of these tests was conclusive, but went ahead and replaced the entire apparatus and that solved it (this was some 20K miles ago, no problems since...)
Leave the O2 sensors alone for now (that, btw, amazon has them for $60 bucks) and run from whoever tells you to replace them. Also run from whoever tells you it's the air flow meter: again it doesn't brake and when it does the car runs rich, not lean.
2003 BMW Z4 2.5i
2009 BMW X5 xDrive35d
2006 Ducati 999S
1999 Ducati Monster 750
Last edited by lpcapital; 01-23-2013 at 12:39 PM.