Engine is running code p0420 and the light only turns on when the cat starts to go bad

How can I check?

How much oil are you burning?

I’m going through about .75 quart a month but I’m not sure how much of that is being burnt vs just leaking out. There is a noticeable oil leak on the passenger side of the engine.

How much are you driving in a year?

About 17-18k miles

I’m pretty sure that it is a cheap aftermarket cat. The shop I’ve been going to charges about half what other shops have quoted me for the fix.