oxygen sensor code P0134
 

Hi I am a newbie.I just bought this automatic 2000 s4 with 70k mileage.
When I bought the car the owner mentioned check engine light was going on and off.
He took to dealer and they said oxygen sensor had a fault code P0134 due to bad gas or malfunction.
I experience rpm's lowering and raising with foot steady on gas under 50 mph in drive.
No problems with acceleration until I have reached the speed limit where I am driving.
I can accelerate to 35, 45, 50 and have no issue until my foot is steady trying to maintain those speeds.
If I switch to tiptronic and drop down a gear there is no rpm raising and lowering while trying to maintain those speeds.
Maintaining speeds over 55mph in either tiptronic or drive with foot steady on gas I have no rpm problems.
I drove over 250 miles with that check engine light on paying attention the oil and water temps with no problems.
There is also a strong gas smell. Are these charactoristics of faulty oxygen sensor?
If so...How do I get to this sensor? Is removal of front required?
I am handy with wrenches and would like to do my own work.

Also new problem today unable to start car in park. Put it in neutral and no starting problems.

I plan to purchase a Bentley service manual.