This bulletin applies to 2020 GMC Sierra trucks equipped with a 6.6L L5P engine.
A customer may complain that the exhaust brake does not seem to work. You may find no failure codes set. Check for any TCM or ECM calibration updates that may be available.
Automatic downshifts will not occur if the vehicle is in Range Selection Mode. The exhaust brake only activates when the transmission torque converter is locked. This can vary based on vehicle speed, gear and load.
To activate the system, press the Exhaust Brake button on the center stack. A light in the switch will come on when the exhaust brake is activated. The switch must be pressed at each vehicle start for the system to be active. The driver information center (DIC) displays the message Exhaust Brake On for approximately three seconds, then clears. The exhaust brake will be more active when in Tow/Haul Mode.
