N00b question:

Do all Evos have 2 O2 sensors...one in the O2 housing and one further back in the exhaust? I am asking because the Megan O2 housing I have HAS an 02 threaded port, but it is blocked off with a threaded cover. I was wondering if I need to use the 02 sensor in this housing or if the one further back in the exhaust is OK. I'm guessing I will have to use both, but I thought I'd ask first

 

the second one really isn't need but if u have a cat delete engine light will turn on the one in the o2 is the important one thats the one that u need

 

how can you tell which one needs to be replaced?