I seem to have the same issue. I need to change my O2 sensors but can only find two. Advance Auto tells me i have four and a ford service man said i only have three. But i am only able to find two of them before the cat on the Y Pipe coming from the exhaust manifold. Nothing after.

Can someone help me?

 

You probably have an OBD-I (not II) compliant truck. There is no sensor after the catalytic converter.

Check the under-hood sticker and see what it says about OBD-I or OBD-II

 

I'm interested if this was the case for your own truck, or you heard that's how it works. Because my friends '06 5.4 throws the 'cat under threshold' code after car removal and no amount of clearing the code or driving for the last six months will get rid of the code. I told him he'll need a tuner to get rid of it. My '11 6.2 with no cats will throw the same 'cat under threshold' code if run with any tune that doesn't have the rear o2's turned off.

I'm reasonably sure that O2 foolers were plugged in on my '03 5.4, but I'd have to crawl under to confirm, I de-catted it nearly 10 years ago now. Sometimes I have a hard time remembering what I did yesterday .