DTC C1646-08: CAN Message Failure - TCU: General Description
WARNING: This page is about a different car, the 2016 Hyundai Sonata and 2015 Hyundai Sonata. However, it is still accessible from the selected car via links, so may be relevant.
Through the CAN line, the EPB ECM receives data from TCM such as Gear position and etc. and then operates the EPB system.