Volvo Repair: 2001 Volvo C70 Turbo Coupe, mass air flow sensor, air flow sensor

Question
The check engine light came on.  Our Volvo mechanic informed us that the oxygen sensor and a canister vent line were the culprits.  Both items were replaced.  We drove the vehicle for one day, approximately 40 miles, and the check engine light came on again.  Now  the mechanic says the mass air flow sensor is the problem.  He also says that the 02 sensor problem from the day before must have been keeping the mass air flow sensor from sending its codes to the computer.  Does this make sense?  In two days we have spent $900 on repairs.  We are concerned that our mechanic (new to us) did not diagnose the problem properly the first time.  We're now second-guessing our choice of mechanics.  Any thoughts you have are appreciated!

Answer
Chuck, sounds like he did it right.  It is one of those situations that it will give you a certain code and you run the tests and they come out okay so basically we are relying on the computer to tell us what is going on.  Did he replace both sensors or just the one?  Roger