The CAN (Control Area Network) bus is a simple 2 wire connection that runs between all the major electronics in the car, it consists of a CAN high and a CAN low wire. It allows all the electronics to talk to each other so they know what each thing is doing.
The CAN bus does a quick diagnostics on startup to make sure that all of the CAN bus nodes are functioning. These include the SAM unit, ECU, ESP and ABS controller, Airbag controller and the speedo.
If there is a fault with one of the nodes because of a failure, fuse blowing or damaged wire, a single line will appear on the LCD of the speedo. If you see this, thare is a chance your car won't start.
Failure Speedo Symbol
If there is a break in a wire, a connector is loose or power is lost to a part of the electronics, you will see this symbol flashing on the speedo where the gear number is normally shown.
You may also see the fuel gauge and temperature gauge flashing depending CAN bus fault is.