I have an engine icon on, used a code reader with P0455

P0455 is a leak in the evaporative emissions system. This will not affect fuel economy, by the way. The most common evaporative emissions system failure is a loose gas cap! Ensure your gas cap is tight enough. Beyond that, testing of the evaporative emissions system is required. Sometimes a smoke vapor is used to see where a leak could be occurring. Hands on testing is needed with proper GM diagnostic test equipment. www.repairpal.com/repair-shops