02 x 2?

I was thrilled yesterday when the Check Engine late came on again on the Tacoma.  I just spent several hundred dollars a month or two ago having one of the oxygen sensors replaced.  From what I can gather there are two O2 sensors on the exhaust system, one before the catalytic converter and one after it.  I am wondering if the rear sensor is now acting up. 

There is a way to get the diagnostic code by simply using a paper clip to short two pins on the diagnostic connector.  Doing so causes the code to be revealed via a series of flashes by the check engine light.  I used the same procedure to get a code from the 88 Vette a few years back.

I think I may try to diagnose and repair this one myself instead of spending several hundred bucks needlessly.