I have quite a few mods on my ‘05 S and one item is a Miltek header and exhaust and after installation I keep getting the “catalyst under threshold “ when I let off the gas just a slight. It’s just annoying clearing the CES light so I bought a defouler that has three different bushings that can be installed. I chose the smallest one and so far the light has not come back on so it’s doing it’s job but have also noticed it feels as I lost some power. So could the small bushing be causing it to lean out reducing power?

 

If the O2 sensor for the catalytic converter affects your air/fuel ratio, you have some weird electrical gremlin.

 

The rear 02 se sensor does not effect driveability . It's only there for emissions .

 

What Adam is saying is that the "post cat" sensor just montiors the "health" of the cat.....However, never under estimate the ability the rearend sensors....

If the "seat of your pants" is sensing a loss of torque/power, and you've done nothing else in this fix, back track and remove the spacer and see what happens....