Hello Tango,
You are suffering from the dreaded 171 lean run. You are experiencing a vacuum leak, and need to look for a vacuum hose that is off or damaged. Also a intake or throttle body gasket can also cause this issue. Your O2 is functioning properly because it has detected this issue. Do not replace it.
If the leak is not obvious, a smoke machine can be used by forcing pressurized smoke into the intake system through a vacuum hose or the throttle body and the leak will appear then. Most reputable shops will have on of these. With out it, the alternative to finding the leak is dangerous.
So, your looking for a reason that could create a vacuum leak. Do not use any flammable aerosol's to locate the leak. It can start a fire and cause injury or death!
Scott.
ASE Certified Technician
Ase certified, 24 years, bumper to bumper all makes