Catalyst System Efficiency Below Threshold (Bank 1)
is this the O2 Sensor going bad???
or is this telling me I may have a cat going bad???
code is from my 99 Yukon

 

bad cat more than likely

 

arent the sensors interchangable? maybe try swapping the sensors around and delete the code and see if it comes back on the other bank. that should tell you exactly what the problem is

 

I fairly sure thats a cat code not a o2 code. Mine threw that for a while from the cats bein high flow so i had it turned off from the tune.

 

the rear 02 sensors are what monitor the cats, so either way its an o2 sensor throwing the code...the real question is if its a bad o2 sensor or a bad cat. its a tough call since the o2 sensors like to go bad alot.

 

That is true. Didn't even think about it. But if they are stock cats they can go bad fairly easy as well.
Swapping the o2's around would be the best way to find the problem.

 

or gutting the cats and deleting the codes im sure daniel isnt into the idea of modding the family truck tho

 

im not sure where it is, but i have a spare rear 02 sensor layin around somewhere. if your prob is the o2 sensor, its all yours if you want it daniel. problem is finding it

 

Like he said ... the o2 sensor is throwing the code, but why is the real question.

I'd try swapping them out, if you still get it, then it's the cat.

 

Either the o2 or the cat