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  1. Duramax Sixth Generation: 2017-2019 (L5P)
    My truck is a 2018 GMC Sierra 2500 6.6 turbo-diesel with an Allison transmission. I believe that makes it an L5P. It has 104K miles on it. Received warning light that I needed to add DEF or my speed would be limited to 65 MPH. The next chance I had, I bought some DEF and tried to add it...
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