There are 2 O2 sensors. The second one is read to make sure the readings change after the primary CAT, which is on the stock header. Aftermarket headers do not have a CAT in them, so you will always have a CEL. A couple of spark plug antifoulers can be used to pull the second O2 out of the flow a bit to trick the ECU into thinking a CAT is present between them so the code is not set.

If your mechanic thought the O2 was bad due to the code being set, you probably bought an O2 for no reason at all.

 

yup and u dnt want to have a cel on all the time ur gonna be running rich or in other words limp mode

 

yep im a ttc and i get the same thing because i have no cat just get a spark plug antifouler it will work they come in a pack of two on the little spinny rack at checker or auto zone and it lie 7.00 you drill one out so the 02 can fit in it and screw that one into the secound one, that you did not drill out this should get that code to go away and if it comes on just re set the batt. it takes like two secs and your car will be happy

 

Well the O2 sensor was actually pretty burned because whoever had put it on previously (since i bought it from another owner) had overtightened it. so just buy a antifouler and get it installed and problem solved?