What is the exact code that the mechanic read? P???? is the format.
If it does point to an O2 sensor, you or any muffler shop can replace it. Takes 15 minutes and most of that is waiting for the exhaust system to cool down.
Bosch makes em for Porsche to label. I used them. Get the ones where the pigtails are crimped on at the factory. Not soldered on and you can't do as good a job of crimping.
Do not use "universal" or off-brand parts. Too many problems reported.
Bosch part numbers are in this posting
http://sites.google.com/site/mikefocke2/oxygensensors though the parts sources and prices are surely out of date.