Volvo: 2002 Volvo XC70, vacuum leaks, oxygen sensors

Question
My '02 XC70 has been running just fine. I had it in just recently for a oil change and check up.  I've got about 53,000 miles on it.  Out of the blue the check engine light came on.  I took it to a shop and they said the code that came back on it was "P0171B Bank1 system too lean".  They said that they checked for leaks, found none, and said that one or both of the oxygen sensors needs to be replaced.  (Cost of almost $350 bucks!) I get the feeling that they don't really know what is causing the check engine light to come on and hope that by just replacing the sensors it would take care of the problem.  Do you know anything about this specific code and what might be the cause of it?

Answer
Patrice,
Running too lean could in fact be the fault of the H02S. Now if the shop checked for vacuum leaks and found none, checked the fuel pressure, and ruled out the MAF and condemed the H02S by watching the values on the scanner in monitor mode, then that is the part that will fix the problem.