O2 Sensors

I have an 04 GTO, 4-speed automatic with stock exhaust. How many total oxygen sensors are on this thing, 4? What is the setup? 2 upward, 2 downward? Just curious if anyone knows. One of them is throwing a code.

Thanks guys!

Yes, 1 pre cat, 1 post cat both sides for a total of 4 sensors.

Ok, so which set of oxygen sensors is the "upstream" and which is the "downstream?" They both appear to be pointing downwards....

Anyone?

Ok, so which set of oxygen sensors is the "upstream" and which is the "downstream?" They both appear to be pointing downwards....

Anyone?



"upstream" = Before the cats, closest to the engine.
"downstream" = After the cats, furthest from the engine.

It doesn't mater which direction they are pointed, just the location.

Cool. Thank you very much for the help! My O2 sensors are throwing a code and there is a huge difference in pricing between the upstream and downstream sensors. I figured I would try the cheap ones first and see if it fixes it.

Thanks for all of your help! It has much appreciated, guys!

Cool. Thank you very much for the help! My O2 sensors are throwing a code and there is a huge difference in pricing between the upstream and downstream sensors. I figured I would try the cheap ones first and see if it fixes it.

Thanks for all of your help! It has much appreciated, guys!


I bet its the one after the cats. Do you have a way to check which code?

Yeah, I can go to AutoZone and find out what error code it is. There is a different code thrown for upstream and downstream huh?

I will try and get the code tonight. Thanks for all of your input!

Every once in a while you can find an owner who is selling their used 02 sensors because they went with catless headers. I got one from a fellow 04 owner off another board for quite less than new. Look around you may be able to find one relatively cheap. Yes the rears are more expensize than the fronts unfortunately.

Okay, that brings up another question. I live in Illinois, this car is a daily driver, and I have researched exhaust options. I have been told that once you put aftermarket exhaust on, theres no longer a spot for the O2 sensors after the cat. I suppose you would disable them using a tuner?

Anyway, so if you put catless headers on - does that disable the pre-cat O2 sensors too? Wondering what options I have because I know I have to go to Illinois emissions test every 2 years too.

Say I got an aftermarket cat-back and disabled my post-cat O2 sensors....when the emission testers plug into my OBC, is it going to cause any issues?

Thanks for your input guys, this is the only place I get certain answers.