GM recently issued a new service bulletin to all its dealerships outlining what their service departments need to do to counter their warranty exposure due to modified programming and performance equipment. We've suspected for quite some time that GM was able to detect modifications, and now we know they are educating their service techs in ways to do just that. Here's a quote from a new article that discusses it.

GM blames after-market software for Duramax problems

In a Technical Service Bulletin (TSB 08-06-04-006J), General Motors sent updated information to dealership service departments on identifying non-GM engine control module (ECM) calibrations. The TSB also extends the procedures to cover any Duramax-equipped vehicles built for model year 2013.
Read more: http://www.torquenews.com/1080/gm-bl...ramax-problems