Is that possible?!

 

If the cat was really really bad, I guess it could. Not likely but definetly possible

 

Did you get a bosch o2?

 

If you've got no catalytic converter an both 02 sensors in, maybe the 2nd 02 is reading that the engine is running rich and leaning it out. Removing the front 02 is causing it to run richer (staying in open loop), and the engine is, or seems to be, running better. Its a theory, don't know if its right or not.

 

I dont think the second O2 has anything to do with fuel mixture. I'm pretty sure it's just there make sure you don't scrap the cat

 

both are ngk (ntk)

 

With all these probs, I'd at least eliminate it from the fold.

 

What's the best way to tackle possible shi+ in the muffler?

 

Screw the O2 back in, and use a pressure gauge on the exhaust, in front of the cat. Should be less than 2 lbs at idle.