I see that everyone is saying to use the 94106 cats from magnaflow.
I have a 2001 4x4 with a 4.6 that has just over 200k miles on it. Still runs great.
I am going to remove all 4 cats and reinstall one high flow on each side.
My question is what about the O2 sensors? If I don't want to do any O2 mods, I thought I would need to use something like the no. 94056. It appears to have the O2 hole in the middle to reinstall the probe.
Please give me some feedback.
I am also looking at using a 40 series flowmaster, no. 42541, which will put all the exhaust at 2.5" as well. I am hoping to help this old girl breath a bit better. As I said, its still runs real good, you'd never guess the miles but I figured its time for a few upgrades. I am also going to replace the old Goodrick TKO's with somethng different (cheaper also). Looking at Toyo Open Country AT or Firestone Destination AT. Either way I am looking at about $640.00..size.... (265x70x17's)

 

The 94106 cats would work well, the shop just needs to mount the O2 sensors after the hi-flow cat. It will work and keep the engine light off.

I would also recommend not going with Flowmaster, go with a Magnaflow muffler to help it breath out better.