I get this code from time to time (P0401). What exactly does the EGR valve do? Seems that it will throw this code if I let the truck idle for say 10 minutes or so. I do mostly city driving, so there's a lot of starting/stopping. Should I be concerned? The error seems to go away after a day or so. I've called GM about it- dealership tried to say that powertrain wouldn't cover it, but GM said otherwise and gave me a claim number. I just haven't had time to get it to the dealership.
Thanks.

It recirculates spent exhaust gasses back into the intake to be burned again, and helps with emissions. Atleast in theory. Really, it just coats your intake with oily crap that robs performance, and the system never really works right on diesels. GM doesn't really care, and they also have problems with it.

You can permanently block it and the code, with an EGR blocker plate and get a tuner, or someone with EFI Live to block the codes...