Theory Of Operation
WARNING: This page is about a different car, the 2016 Dodge Durango. However, it is still accessible from the selected car via links, so may be relevant.
The purpose of the diagnostic is to monitor the Controller Area Network (CAN) bus, the transmitted messages on the CAN bus, and the validity of the CAN signals. Several control units are connected to the CAN bus which send their messages cyclically in a defined time window. The messages contain multiple signals representing vehicle information.
CAN Signal Monitoring: If the sending control unit does not have valid information to send, it may send a defined failure value indicating an invalid signal. Incoming signals with a defined failure value are monitored for signal validity. For some outgoing signals, the Transmission Control Module (TCM) reads its own signal back from the CAN bus to monitor the signal transmission for disturbances.