EGR explained

I finally got my check engine light to go out. For the longest time I was getting a check engine light come on when coasting at highway speeds. Codes were 33, 34, map voltage too high and map voltage too low. I took the car to
a GM goodwrench garage. They told me that the EGR valave was not functioning. I took the car back to the garage that I bought it from and asked them to replace it, since I just bought the car 1 week ago, I figured it should be their responsibility. Anyways they were very hesitant, and said it would not fix the problem. But I told them regardless if it fixed the problem, they still sold me a car with a non functioing EGR valve. So they replaced it with a used EGR valve, and when I took the car out on the highway the first time the light did come on. I let the car sit for 2 days without driving it, then the past 3-4 days I have tested it extensively and the problem seems to be fixed. Before it was very easy to get the light to come on, just coast at highway speeds for a few seconds and it would come on every time. Now I cannot under any circumstances get it to reproduce the problem, so I can only assume that the EGR valve replacement has fixed the problem.
That siad, could someone please give a brief explanation of how the EGR valve works ? Is it likely that the faulty valve was causing the problem I was having (map sensor voltage too high/low) ?
Just trying to understand how it all works, and if I should feel confident that the problem has actually been fixed ?
Thanks for any responses.
Regards, KDawg
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Nice little article at this link http://www.mycarwizard.com/autoshop/exh.html

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Great site. Everything laid out in simple terms for a non mechanical person like myself.
Regards, KDawg

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