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Certain 2018 model year Buick Regal vehicles may have a condition where they were built with rear differential modules (RDM) which have one suspect bolt hole with a shallower depth than the rest of the bolt holes, causing the one bolt to bottom out and not fully seat. This can result in the RDM cover becoming loose, which could result in fluid loss and diagnostic trouble code (DTC) C0407-64 setting. The AWD SERVICE message could be displayed on the driver?s information center, (DIC). Replace rear differential module (RDM).