VSC light 1999 Lexus es300

OK: This time when the VSC lights came on the Check engine light came on also. And now stays on even when I restart the car. I went to NAPA used their scanner (for free) Thanks; and got the Code PO420. The same code I got before Lexus changed out the gas cap. The gas cap “fix” worked for 6 months??? If it was chump change to replace the cat I do it, but I would like to know that my $1500 is going to give me a perm fix.
I apprecieate your thoughts.