Changed the Fuel Cap but light came back on!

Question

1998 Chevy 5.7 ltr automatic 157,000 miles

Answer 1

HI there,

P0420 is for the catalytic converter on bank 1. It's not working properly and may need to be replaced.

With the P0440, this is affecting the whole system. If the new gas cap didn't work, then you should do a smoke test to locate the leak and then replace the faulty component.