Ok, a couple weeks ago, my check engine light came on. I finally got around to finding out the code yesterday, and I got #15 which, according to this site, means I have a bad oxygen sensor. I have a '91 coupe, with a streetport, 2.5" downpipe w/ presilencer, and HKS exhaust system. I was wondering if these mods would make it think it has bad O2 sensors, when it doesn't. Since I have a streetport, I am also thinking of getting an S-AFC II to fine tune it. So, if I do have bad O2 sensors, would I still need to get new ones if I got the S-AFC II?

 

replace the o2 its like $20. this one can be caused by big vacuum or exhaust leaks too, but its more likely to be the sensor