DML@ME

Just had this same turn signal failure warning occur on my 2018 Sierra with factory LED taillights. The light actually worked in all functions but would register a warning after the fourth flash. Deal went about trouble shooting and back to GMC for guidance because it all tested out and no codes in the control module. 3 hours later and un-spooling the wiring harness they found a corroded / chafed wire in the harness from the engine bay to the rear of the truck.