So, let's talk about exhaust & O2 sensors. Are there 4 or 6 on a 2000? I thought there were just 2, at the cat on the header. These are the 2 (red) that I have replaced. Is the 3rd (blue) really there and if so, what is it's purpose? Looks like it is on the secondary cat. And if there are 3, how come only 2 show up in any tester?
Assuming for a moment there is clogged cat. Would it most likely be the one at the header or the secondary cat? Waiting for pb blaster to soak in to get the front o2 removed on the header so my mind is just starting to think 10 steps further down the list.