If your Duramax is a 2008 mdel, it probably has the diesel particulate filter (DPF). Those things can be somewhat finicky -- especially with the electronics, sensors, etc.
I'm not familiar with the Duramax in particular, but I drive heavy trucks (Freightliners, Sterlings, Internationals, etc.) with much larger Cummins or Navistar inline-six engines. All of ours that are post-2007 models are equipped with the DPF and we have had a lot of trouble with them.
There is a bypass/delete kit available for the DPF and it will improve your fuel economy, but that is illegal and you could jeopardize your warranty.
"If I passed you on the right, you're in the wrong damn lane!"