Just bought an 07 lbz. New to duramax world. Truck ran fine for 1 month after I bought it. I drive it easy. It's stock besides a 4" exhaust. Truck went into limp mode as I was driving on a straight away accelerating easily. It got stuck in 3rd. So I turned it off and cycled the key a few times and went back to normal. But went back into limp mode about 30 minutes later. Driving it easy with normal acceleration. Why is this happening?

 

Are you getting a check engine light with codes?
If your in limp mode, than you should be getting a check engine light with codes.

 

Yes my check engine light is on. It stays on because of my egr has been blocked and i haven't got a tuner on it yet. I'm going to take it to a dealership and get them to diagnose the problem.