About half a year ago I had to replace the front part of the exhaust on my 2003 Corolla. SInce on this model it is welded to the cat converter, it had to be replaced too. Shortly after my local mechanic fitted the (aftermarket) unit, the engine check light came on. I brought the car back to him, he ran some emission tests and said something along the lines of the ECU being too sensitive to the tiny variation in the levels resulting from the aftermarket part, and that unless I want to pay ~£1000 for the original unit, I may just have to live with this. I was too busy with other stuff at the time to argue and so I just went on with it.
Yesterday I brought the car for the MOT to a different garage and it passed with no advisories. The exhaust issue was noted though, the codes cancelled, but after 5 minutes of motorway driving the engine check light came on again.
So I'm still wondering whether it's a good idea to just live with this. Does the ECU put the engine in some special mode once it detects cat issues? Is this a problem to always drive in this mode? Also, if I replace the exhaust+cat again with another aftermarket unit, what are the chances that the problem will remain? Or should I try replacing the oxygen sensor with a (possibly less sensitive) aftermarket unit as well?