2001 Hyundai Santa Fe Code P0421

I've been having trouble with code P0421... I replaced the catalytic converter and it went away. Two days later the check engine light came back on so I went back to the mechanic and he cleared the code... then 3 days later the check engine light came back on again. I don't know whats going on... could it be that the oxygen sensors are faulty? Also when I come to a stop at a red light somewhere... sometimes the car feels like it jumps... it doesn't move forward but it just feels like some sort of bump. It runs fine but the only problem that comes up is code P0421... anyone have any suggestions? Thanks.


--I re-posted this because I accidentally posted it in the wrong section.

Whay kind of converter did he install? The bump you are feeling is possibly a misfire and this will damage a converter over time.

how do i fix the misfire?
... and im not sure what kind of converter they installed... what kinds are there?

There are cheap, fits-all cats that skimp on the precious metals needed to do the job and last almost as long as a burning cigarette and then there's cats made for your car.

Normally, with cats, you get what you pay for.

The bump MAY be a misfire...address the cat issue first.