Smog test says unset drove over 300 miles still says unset

Hello, Dave here, contributing to RepairPal and providing expert advice since 2010. If your engine processor is still setting the check engine light with an active code of P0420 - then there is still a problem present. Is this a 4 cylinder engine equipped vehicle? Was the catalytic converter an original equipment General Motors cat? Were the oxygen sensors original GM parts? If the light is still on, a fault remains. It's not about driving the vehicle at this point. The light and P0420 code indicate an active issue that still must be resolved. Spend some time with a technician at a shop near you to evaluate. Note that if aftermarket non GM parts were used, it could be a problem. https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.