Installed the O2 sim today, since I have a CEL that came up after install of the new highflow cat + catback system. After installation, the sim blinked as supposed to, but the idle of the car had become very rough... ?? The narrow band A/F gauge indicates very lean (?) sometimes it even goes off the chart (very lean). While driving the car all seems (almost) normal. it's the idle....
So I put the whole thing back as it was before, but the idle is still rough, nothing seems to have changed.

What should I do ? reset the ECU, by disconnecting the battery overnight ? I have OBD II with this 2000+ RR-motor. Of course the CEL has not gone out either. tomorrow I'll take it to a shop to have the CEL diagnosed too, you never know.

Any advice ?

Don't post the same thread in multiple sections.

Thanks for the advice....