Failed smog check read error code P0421 but no check engine light.

Question

I recently brought my 2007 Mazda 6 in for a smog check. Smog check failed with the obdii reader, reading a p0421 error code but I currently have no check engine light on. I just recently changed the battery and got a oil change.My first smog the technician asked if I replaced my battery which I did. He advised me to drive around for 100- 200 miles. Drove 200 miles and brought it back in for a second smog and at that point he notified me of the p0421 error code. He then advised me to drive around a little bit more before i take in for the third time.