The computer is not saying it needs a new converter. The computer says things like "the resistance measurement from this or that part is higher or lower or inconstant etc. Then the so called mechanic just replaces this or that without bothering to see exactly what the problem is and what might have caused it.
In short, it sounds like you need to replace the mechanic. BTW Dealers are no better or worse than independent mechanics, but dealers are almost always more expensive (got yo pay for that fancy coffee somehow.)
Final note, Fast oil change places do not attract mechanics, but they sure attract some shady salesmen.
It might not be easy to do it from here, but if you give us some more data, maybe someone can help you out. Start by getting the actual codes that were read. Even if the light is not still on, you may be able to get the codes anyway (sooner the better) The code will be in the format P1234 Many auto part stores will read the codes for you for free.