Why is my GMC sierra lane departure warning not working?

If the lane assist for Your GMC Sierra is not giving You departure warnings or is telling You it's not available the first thing You'll need to do is take in to a dealership. These trucks originally had an issue where this feature didn't work properly but it has since been fixed with a system update. This update will be free if You are still under warranty. If You've gone to get the update or had it done previously then there is going to be something wrong with one of Your sensors and You'll need to take it in to a mechanic or use a scanner Yourself to narrow down what the issue is.