Originally Posted by foaxaca
Decided that it wasn't a fair test to try the +4 plugs with the pre-cat O2 sensor leads reversed. Switched the leads and reinstalled the +4's this morning and the car started, reached proper idle and ran fantastic. No misfires, no CEL and great power. Smooth and strong acceleration. I'm going to leave them in. Is it safe to assume that if I am not getting any post-cat O2 sensor codes, I shouldn't worry about changing them out also? Have about 3/4 tank of fuel with SeaFoam in it I have to finish off so I have some time left before retesting.
My opinion is to leave them in there if you're not experiencing any problems. You bought some pricey spark plugs, it be ashame to throw them out at this point.