I have a O2 CEL because I put in the Megan headers. I know how to simulate it and get rid of it.

But what I was wondering will having the light on effect performance?
I haven't notices a difference.. Just wondering..

 

The car may go full rich, it may pull timing to prevent any damage, it may even go into safe/limp mode. One way or another, give it some time and it'll run like crap.

 

is it a P0420 code?

 

Working parts retail for 2 years, I've seen that one plenty of times and have always liked people's response. "What does 'Catalyst Efficiency Below Threshold' mean?"

 

Correct, all the second O2 sensor is for is to monitor the efficiency of the Cat. Converter...Now the effect of this fault on engine mapping is unknown to me. Like said earlier, pulling timing etc... Stick that simulator in there and the DME will use the signal of the front O2 just fine.