Well as the list says, it could be a bad O2 sensor, but you do have to do some checking and make sure that there is no underlying cause to the lean condition. That is to say is the O2 sensor just reading incorrectly (therefore defective) or is it reading correctly (therefore not defective, just reporting that it sees the A/F as lean due to some other cause).
Unless you have a misfire code stored, which it does not appear that you do, or if you have seen your check engine light flashing which I suspect is not the case you may and I stress may possibly rule out a misfire condition. That leaves air flow or fuel flow issues.
If its either one you are going to notice a reduction in power. Now, that said, it may be helpful to post up year, make and model and what if any maintenance has been done recently, how many miles, etc.
A couple of things you can check right away would be:
1. The MAF sensor. Maybe get some electronics cleaner and clean it good and the connector. Do this particularly so if you run any kind of oiled air filter.
2. Check for vacuum leaks.
If those are all good and dont clear the issue, then perhaps replace the O2 sensor and see if that corrects the issue, or you can start there first if you like, but I like looking at the really simple things that are easy to take care of first when possible.
