08 Gran prix 3.8, p0402 & p0420
402 is a prob with the egr system. could be a dirty or faulty valve, or even a differential pressure sensor. 420 is a catalyst efficiency code. most common is a bad/plugged cat or bad o2 sensor. can also be damaged or leaking exhaust or wiring to the o2 circuit.
btw, check the egr system first. if there is a problem there, resolve it first and turn off your codes and see if the 420 comes back . sometimes the first code listed can be what is causing other codes to happen.
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