Giulietta QV downpipe question
I've been thinking of replacing the very restrictive looking cat and downpipe on my Giulietta QV, with a custom build one with a 200 cell cat.
There is a lambda probe (or o2 sensor?) in the downpipe, right in front of the stock cat. On the new cat I'm thinking of buying, there is a hole for that as well.
But, there is also a second lambda probe, after the cat, right? I've read that this can cause problems if you change to a higher flowing cat or go completly cat-less, such as only being able to run the car in N and getting the check engine light.
My question is, what is the purpose of this second probe? I've read that you can fit a lambda faker on it or simply map it out of function. Will this affect fueling in the engine? And will the car still be able to pass an MOT?
It would be nice to hear some experiences from people who know about this or have changed the downpipe on their own car :-)