I remember when I was confused about the O2 Sensor.
My 911 was failing smog, and I had to dive in and learn the system, then try to troubleshoot the problem.
The worst part was that the O2 Sensor plug was broken by the shop that was supposed to fix my 911.
Here's a pic of the two circuits for the 3.2 911 O2 Sensor. The plugs are located where Ed says, on the left hand side below the fuel filter.
I was able to glue the plug back together. Now I hold the O2 Sensor onto the plug with a zip tie around the rubber boot. I cut the rubber boot lengthwise to make it easier to remove next time without breaking the yellow plastic plug.
