Pass emissions inspection-next day service light is on w/ P0421-how did it pass?

Hello, emissions testing is performed and results are based off of what is happening at the time your vehicle is being tested, as well as what is in "history" in recent times. Apparently, all went well for you. Although this trouble code can indicate a catalytic converter is not working as designed, it can also mean other things and warrants hands on testing. www.repairpal.com/repair-shops