zeroptzero

Hi, in an attempt to lower my hydrocarbons reading during emmissions testing on an 02 S2000, I have a newer primary 02 sensor to swap into the vehicle. I want to confirm if the primary 02 sensor (the one in the end of the header) is the one that affects air/fuel ratios the most ?, or do both 02 sensors do that ?. I can only get the primary sensor right now, so I was hoping it might help things out. I'm thinking the secondary sensor in the cat. conv. is mainly to compare emmissions readings to determine when the cat. is compromised. Please let me know if I'm correct in my thinking. If I can get away with one 02 sensor right now it's worth a try. My car has 55,000 miles on it and it was borderline on passing emmissions testing, running overly rich with unburned fuel. Thanks very much for any replies on this.

chansta

id say the secondary sensor affects it the most since it'll read the amount of 02 left over to adjust the AF ratio

MasterForce

The O2 on the header is more important than the one on after the cat. Mine went bad and car wouldnt start for a while and would run like crap at low speeds. once i replaced it everything went away. The O2 after cat wouldnt matter that much anyway because alot of people get test pipes.

zeroptzero

that was my best guess about it, with the primary being closer to the engine it should be in a position to give the best reading.

Good to hear that a new sensor helped your situation, how many miles on the car when it went bad ?.

Thanks for all of the replies.

IIXtremeHansen

wow lots of bad info. Primary is what reads better

Kirpich

Primary just monitors how close the pre-programmed map is to the conditions the car is in (himidity, temp, baro press., etc.).

There's a map pre-programmed, and since Primary O2 is narrowband, it only tells the ECU if it's too rich or too lean. That's it. So the ECU is always in the ballpark, like a guess. The ECU never knows _how_ far off, just if it's over or under. ECU is constantly adjusting, and that's called long-term and short-term fuel trim.

MasterForce

went bad at about 44xxx miles. found out they were around $200 at the dealership and just got one used for $50 on here.

drhess06259

the primary is the one the ecu looks too for fuel curves.... you can't tune the car without it. the secondary is basically just to see if the cat is still working for emissions.

zeroptzero

thanks for all the great replies, they sound reassuring for what I'm attempting to achieve. I also find the price of new 02 sensors to be extremely high, particularly in our neck of the woods, so I've got a low mileage one on the way from a fellow member with 3k on it. thanks for all the opinions on this.

Stratocaster

FYI, the dealership would have replaced it for you free of charge. They are covered for 80K/8yrs unbder the emissions warrenty.