Hello all, I own a 2001 Ford Expedition 4.6 2wd, and I'm a little confused. This car was purchased with cat inefficiency codes when I bought it from the previous owner. So I'm looking to replace the two front cats. What I thought I was going to find was 4 cats in total, but instead I found 3. One on the passenger side, and 2 on the drivers side. Alldata gives the a part that has 4 cats, but I only have three. Perhaps I need to go look again, but I'm pretty confident there are only three. Does it not matter, as long as there's a functional catalytic converter in between the o2 sensors, and that I should't worry? The entire unit looks original, but I could be wrong. Thanks in advance.