I searched the forum for o2 sensor questions, but most of them pertain to obd1 cars. The reason I'm posting is I ran a testpipe for a year now, and when it came time to do the emmissions I switched back to the stock cat. Problem is I'm still throwing a cel. Is there a difference in the primary and secondary o2 sensor? The reason I'm asking is if my o2 is bad I plan on buying one from nopi made by NTK (NGK), but they don't have that part listed for obd2 cars, only obd1. If I bought that o2 sensor would it matter if i had as a primary or secondary. Is there a difference, do you think I fouled out my o2 by running the testpipe. Thanks for reading and helping!!!