Ok I’ve read and searched many threads on this topic but mine is a little different than what I’ve read. I have an 04 5.4 3 valve. I’ve modified it he exhaust and removed the rear cats. Truck ran beautiful like this other than the fuel delivery issue from a bad pump. So I replaced the pump and got new injectors and a new throttle body. Also replaced the IMRC. And put on a CAI. I then removed the front 2 cats and put mini cats in front of the O2 sensors. Now the truck runs rich as hell and bogs and pops on take off. Once it gets going it runs a little better but still rich. And it gets progressively worse as it gets to operating temp. I do have a new cat set up to put on the truck. My question is.... is the rich system due to not having and cats now.

 

It's rich because you have a cat before the front O2. The cat has to be between the 2 sensors.

 

the problem is is when you delete your cats you are pushing more air out than the amount of air coming in. so i would try resetting all of your sensors and computer so it will accommodate air intake and fuel intake to how much is coming out. and for next time i would keep the cats off and just by fake o2 sensors so it doesn't throw a code no matter what. but then you rely on only your senses to tell if the truck is running wrong.

 

And if this doesnt work then i would suggest making sure you put the right o2 sensors in. Such as making sure the one before the cat is labeled upstream and the one after the cat is labeled downstream.

 

There is no difference between the front and rear sensors except the length of the pigtails. The front sensor has to be a real sensor and before any cats or it will run rich.