What's wrong with my car? Can the catalytic converter be the problem?

What is your question?

Engine light is on and has a - PO400 'A' Flow - code. Already changed EGR valve, solenoids and hose within the last month. Many times when pushing on the gas the car remains in a slow pace then slowly picks up in speed with a jerking motion. The mechanic who wants to take a look said that he don't really know about Mazda cars that well. How can I let him look if his knowledge is lacking.