Another O2 sensor question

[cyberegg]

Has anyone heard the terms upstream or downstream along with oxygen sensors? My codes indicate Downstream sensors. My somewhat logical brain is wondering if I only need to replace the rear sensors. But I have no idea if my Disco is logical as well.

I don't have maintenance records, so I don't know if any O2 sensors were replaced earlier.

 

[joey]

upstream are closer to exhaust manifold prior to cats.
downstream are after cats...

If getting downstream errors, I would get an OBDII reader that will show the accual signal to make sure it is working or not....

this could be a bad O2, or bad Cats.