Check engine light on, code reads EGR faulty.

These things can be tested if you know how they work. I don't know which code you had, the code describes the malfunction and that would be a big help in diagnosing the EGR system. I imagine that the EGR and solenoid come as a set but you could call a parts store and ask. The EGR must be allowing too much flow or too little, or it leaks. The solenoid either works or it doesn't. Go from there. If you want a GM specialist, here's a directory link for you: https://repairpal.com/pontiac-repair-in-lancaster-california

I have a grand Prix its a 2007 and my gas cap light and my check engine light both are on changed the gas cap but still comes on engine light says evap went to have smoke test they told me they couldn't detect any leak reset the light leave come back right back on again. Now different cold egr valve or p0403 how does it just change like that ??