CAN is actually a standard serial differential bus broadcast interface, allowing the microcontroller to communicate with external devices connect to the same network bus. The CAN interface is highly configurable allowing nodes to easily connect using two wires.
When connecting external devices to CAN bus in the car or other system, there is always risk of influencing the communication on the bus because of this device. Like elementary mistake as incorrectly set bus speed, if the device is not in Listen only mode, disturbs the communication on the bus, thus the car control units report error and the car can be out of order. Listen only mode eliminates this risk and to figure out which device don’t work correctly! In a vehicle, CAN SNIFFER device allows data reading from CAN bus by means of tapping and signal reconstruction at the bus through its scanning from conductors through their insulation.
To know more about the project check this document PROJET_Sniffer.pdf