Looks like you are getting generic and not spacific GM ones from my codes
P1420 Intake Air Low Pressure Switch Circuit Low Voltage
P1404 Exhaust Gas Recirculation (EGR) Closed Position Performance
So it looks like it could be the EGR valve it may be worth cleaning it and trying it otherwise buy a EGR cheater and a blank plate, if you do a search there is plenty of info on the EGR issues.