Knowing you car is a bit important than relying on orthodox mechanics .
So we start learning.
Stage 0 is catalytic converter this is expensive part and mostly the mechanics tear off the inside ceramics even if the CAT is not choked.
Function of CAT.
https://www.youtube.com/watch?v=v9ju2rZ8YmM
Understanding O2 sensors what they do and how they do.
https://www.youtube.com/watch?v=XQYDwIWS6Ho
how to check the O2 sensors.
Set of series about EFI system will teach you lot of stuff that the best mechanics do not know
this is the third lecture you must watch from lecture 1. you will know what your car is doing and why.
https://www.youtube.com/watch?v=zTK9-DoGIVY