four o2 sensors
 

i am putting in catless headers, and i pulled off my old cats and i noticed there are two o2 sensors on each side. why first, and what i do now because my headers only have one hole for them on each side.

What brand of headers do you have? Most of them I think come with spots for the upstream and downstream sensors....there are two of them to check to make sure the converters are working properly

pacesetter headers, didnt look at stuff to hard, but didnt see two ports. do i need both sensors, im going to get a dyno tune

Usually the tune required to get rid of the check engine light shuts off the rear o2's they exist only to double check that the front ones are working correctly and making the necessary adjustments to reduce emissions. By getting rid of your cats they are pointless anyways. Most headers have them just installed and non functioning. Some remove them altogether.

No, the only purpose of the rear O2 sensor's is to verify the operation of the cats. All cars are required to have a sensor before the cat for engine monitoring/tuning and a 2nd sensor after the cat. The ONLY purpose of the post-cat sensor is to verify the cat is working - there should be a lower O2 reading after the cat than before the cat. With dual exhaust and two cats, you need two sets of sensors.

You can tune out the 2nd sensors, but the easiest (and most warranty friendly) way is to connect O2 simulators to the wiring harness (in place of the rear sensors). They provide a fixed resistance to fool the ECM into thinking the cats are doing their jobs. They are plug-n-play and any performance shop should have them.

clg has it right! The downstream (sensor 2 ) has no effect on the tune. They can be turned off with most tuning software. :thumbsup:

Well since they're no longer legal and the company making them got all kinds of sued you can't find them online. If any shop has some that they want to sell PM me because headers are in the plans this fall but no tune until the cam goes in next year.

I tuned my rear O2's out using EFILive.

Bill

This is what my online research led me to believe. The front O2 sensor is to read how much "unburnt" O2 is left in the exhaust to help control a lean/rich fuel mixture and the 2nd (post-cat) sensor is to detect that the catalytic converter has removed enough O2 from the exhaust stream by converting things like carbon monoxide into carbon dioxide.

And it's for this reason, I didn't really ever believe that having high flow cats would cause you to run lean because the 2nd O2 sensor was measuring cat effectiveness unlike the first one that basically measures the amount of O2 left in the exhaust after fuel burn, which would be the same whether or not you had the stock cats.

Basically, lets say you're running lean and you have a surplus of O2 at the first O2 sensor before the cat, the 2nd O2 sensor should expect a mathematical ratio of O2 after the cat based upon the first O2 sensor's reading. If the ratio is off and the 2nd sensor, then that would mean the cats aren't working, if the first sensor is reading too much O2 then the reading of the second sensor should also read too much O2 meaning you're running lean, but the ratio post-cat:pre-cat should be the same.

To be even more long-winded, I read that the reason post-cat O2 extenders work is because when you remove the O2 sensor from the center of the exhaust stream, the less dense air on the outside edges reads lower O2 thus, fooling the 2nd O2 sensor into thinking the cats are working at peak efficiency.