Circuit/System Description
WARNING: This page is about a different car, the 2018 Buick Envision. However, it is still accessible from the selected car via links, so may be relevant.
When a remote slow-down request is sent from OnStar, the vehicle communication interface control module sends a serial data message to the engine control module (ECM) indicating that reduced vehicle speed has been requested. Once the request is active, the ECM begins reducing engine torque to match the requested vehicle speed and the REDUCED ENGINE POWER message is displayed.