Do not let code readers and local "chain shops" fool you. They are there to aide them in selling parts. Code readers are a generic function of OBDII systems to "AIDE" in diagnosing vehicles. This just lets you know what is being detected.
For example if your system comes in with two codes bank1 and bank2 lean. Your chain shops (autozone for example) may try to sell you a 02 senser to fix the problem, when in reality it could be in the MAF system which has coated the hot wire with sediment and will not allow the computer to read the proper air flow, and that may not even be the root cause yet. Ford, GM, Nissan, Mercury all have publically asked the customer NOT to use a K&N air filter. THe oil from these filters, coats the hot wire sensor with the film of the charge causing issues.
The moral of the story is ONLY use the code readers as a diagnostic aide, they only tell you where to start looking. If you happen to have a Snap On Modis or OTC Genysis (and others) you can look at manufacture specific information and Mode6 data to help you diagnose the problem using the graphing function or oscilliscope.
So instead of playing "Guess-diag" it sometimes is better to let the proper people diagnose the problem, then you can have the option to do the repair. We won't get into "blocking codes" which may come up even after the repair because a blocking code would not allow another monitor to run...
Diagnosing todays vehicles is more complex then one might imagine.
If you should have any questions, please feel free to contact me always willing to help and steer a person in the correct procedure for finding root cause and fix it right the first time.