Quote Originally Posted by Tullphan View Post
Just out of curiosity, if the O2 sensor on the exhaust don't really do anything, why is my gas mileage still down 5-6 MPG? Would a bad O2 sensor on the "downstream" affect fuel mileage?
Someone earlier stated that Bosch O2 sensors were junk. Are they bad from the start?
Like I said, years 01+ have some diagnostic cycle they go through when it detects a defective 02 sensor. This diagnostic cycle causes the car to run rich, killing gas mileage and no one has access to this in the pcm to turn it off. I have a powrtuner, if mine was a 98 or something I could just delete the codes for the 02 sensor when I take it out but I can't with my 01, it goes through this cycle still with the codes deleted.

Just get a 02 simulator. Also, if you look under your car and see that big hump where the rear 02 is right now, you will notice its constrictive. Taking that 02 out and replacing that pipe with a straight one could increase your hp and gas mileage... Its actually one of the first mods everyone does.