Subaru Repair: Oxygen sensor, oxygen sensor, subaru legacy

Question
We have a 1999 Subaru Legacy & keep having trouble with the Oxygen sensor.  Last month the check engine light came on & we took it to a Subaru dealer & they replaced the rear Oxygen sensor.  We had already replaced it once before, but at a local garage so the second time we took it to the dealer to have it properly diagnosed.  It's not a month since the dealer put in a new sensor & the check engine light has again come on & my husband said it is acting like it did before.  Is there something that can be causing the sensor to go bad?  

Answer
There are two types of codes conditional and componet.A conditional code would be P0139 O2 sensor circuit slow response and does NOT mean the O2 is faulty it only means that the conditions have been met to cause the computer to set the code and illuminate the check engine light.The condition could be caused from failure from a multitude of systems.And a proper analysis is needed in that case to accurately diagnose the situation and more than 1/2 of the industry doesn't know how to do that including the techs at the dealer.A component code could be P0141 O2 heater circuit malfunction and is pretty strait forward to diagnose.So my guess, with out knowing the actual code set and without looking at the data stream  is that you have a misdiagnosed conditional code.I know this is a lot of info but i am trying to help eduacate.Its frustrating for you i know but please stick with it.As until it is properly diagnosed and repaired your Suby may be excessively polluting the environment.And i do want to thank you for not ignoring the light as its so important for the health of not only your car but the world we live in.

Justin
All Wheel Drive Auto