I had etc sensor changed, also changed both oxygen sensors and the catalytic converter. The check engine light appears after I drive 20 miles or more. After mechancic changed sensors he turned off light. The code was a p420 cat code but everythng had been changed. Mechanic said that maybe the computer needs to be flashed.

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html

Was the converter replaced with aftermarket or O.E.
You may have a faulty converter.