i had to gut my cat about a month ago it was badly clogged, couldnt afford to buy a new one at the time. truck runs like it should now, my question is, im getting a constant catalyst inefficiency engine code, and i can smell its running rich. i have an upstream O2 sensor and one downstream. i know there is a module you plug in to where the O2 sensor goes, to trick the computer into thinking everything is ok. but do i put it on my up stream O2 sensor (which is in the cat by the way) or the down stream one?
i was thinking the one in the cat so when the computer reads the down stream one it would be different which wold fool the computer into thinking its correct? or do i get one for both, and where can i buy one? thanks guys!

 

The rear one only. The rear O2 sensor is purely to monitor the cat, the upstream O2 controls A/F ratio. I put in a spark plug defouler in mine saturday (same situation you had) and still no check engine light today.

 

Adukart is correct, the upstream will control how your truck runs and you will need a spark plug non fouler to fool the rear. Your truck will always smell like it's running rich without a cat.

"The catalytic converter works by using a “catalyst” to create a chemical reaction in which the gases are “converted” to produce less harmful, or even inert matter. In road going vehicles, this can result in a 90% conversion of bad gasses into less harmful gases that flow through the mufflers and out the exhaust tips."

 

That's not running rich, it's poison gas, that's what the cat is there for. Your local parts store will have the non foulers or you can get one on the internet that don't need to be drilled out

 

awesome, just looked up the non fouler thing, proven it works so im going to do that. thanks guys!