Toyota Vitz (2010) 1KR-FE KSP130 DTC P0420

P0 403 I would attack first and make sure that you do not have an open circuit. It could be an intermittent open circuit heat and vibration . I don’t see how EGR code would set an open circuit code for EGR. Next, I would check for flow EGR system. Most flow problems are not in the EGR itself but in the intake manifold right up by the EGR valve. I hope this helps :-) however your insufficient…

I'm guessing that this is a 2010 Toyota Yaris in USA? Does this even have a EGR since I don't have info on VITZ?

Yes it is same like Toyota Yaris and equipped with 1KR-FE 1000cc Engine.

Because the PCM expects to see a change in exhaust gases when EGR is operational. Since you've you've used what would be an illegal device to cheat emissions, you're getting nailed by the PCM because it does not see what changes it wants when EGR is commanded. A catalyst for a small Yaris here would be in excess of a thousand dollars US, so 400.00 isn't too bad.