I have no idea what you just said lol.. what is a hego? Sorry im a noob when i comes to emmisions stuff

hego means "heated exhaust gas oxygen"

when you mix gasoline and air together to burn them and make combustion happen, there are some important things that need to be right.

think of making chocolate milk by adding the powder (nestle quik) with white milk.

If you add too much powder it will be very thick, very "rich"
If you add too much milk it will be very thin, very "lean"
If you add just the right amount of milk with powder it will be chemically perfect or "stoic"

"stoic" for gasoline and oxygen is 14.7 parts oxygen with 1 part fuel. anything lower is rich, anything higher is lean.

Since the condition of the engine is always changing based on a million different things the sensor after the exhaust manifold is there to read the AFR, or Air to Fuel ratio to determine how close your truck is to 14.7. If it dosent see it bouncing around 14.7 then it makes an adjustment to what it needs to so that it can hopefully correct it some.

The oxygen sensor (hego) that is in the back BEHIND the cat simply looks to see that the Richness of the mixture is LESS RICH after the catalytic converter than it was before it went through the cat at the first hego. This tells your computer that the catalyst is present and working correctly.

There are more details around this but basically this is how any modern car works.

Gotcha. SO Could I take both cats out, and put the o2 sensor back into the pipe? Or do i have to have the cat?

Think for a second about how the system works. The exhaust stream is a 1 way road. The sensors in the system can only look directly at the exhaust going past them, so if you remove a converter that is PAST the last oxygen sensor the computer will not know, and because of this you wont throw a code for "catalyst inefficient" (420 or 430 usually)

If you are removing the cats for performance than you will be very dissapointed. If you are removing them because they are clogged than this will fix your problem. If you put the sensor back directly into the pipe without using defoulers than the computer will see the same signal coming from both HEGOs and you will throw a code (420 or 430 probably) If you use the evo/sti defouler trick (shown on that honda forum above) the rear sensor will see less fuel past it and this will fool it to thinking the catalyst is present and working correctly. I have done this on hundreds of tuner cars and also for friends that had problems with thier converters for one reason or another.

Note: some vehicles with EGR use some sort of sensor that monitors exhaust pressure to trigger the EGR valve. This can be complicated by catalyst removal and can cause idle or drivability issues (partly this is why I never buy a vehicle with EGR if i'm going to mess with it)

all you have to do is buy the weld in o2 sensor fitting and put it into the pipe for the upstreams ( you need those for a/f ratio) but downstreams only tell the computer if there is a cat there so yes there will be a CEL. unless you have a bama chips tuner. which he turns off all downstream sensors. personally i would do it. i did it and love it. i have no cats running to a single flow 40 delta with dual out and i think my truck sounds amazing. i always get compliments on how it sounds. but if you dont have a tuner then you will have a cel.

oh and also my truck runs perfect with no problems. If you were thinking it might run like crap haha