As the ECUs are all linked, a fault in one can be transferred down the line to another e.g. engine management light on but the fault is in the ABS ecu. Unfortunately, this technology is so advanced that drivers have no understanding and why some faults are so difficult to fix.
I have copied the following to give an idea of what it means in nice simple terms ;)
CAN Bus is a multiplexed wiring system used to connect intelligent devices such as Electronic Control Units (ECU's) on vehicles, allowing data to be transferred in a low-cost and reliable manner. CAN means 'Controller Area Network' and was developed by Bosch in 1980. Most new vehicles use this system and it is becoming increasingly difficult to install after-market products without using a CAN Bus Interface.
Why is this system used?
The reasons for the vehicle manufacturers using CAN Bus are:-
(a) Significant reduction in wiring leading to:-
(i) Significant reduction in manufacturing cost and hence retail cost.
(ii) Reduction in weight resulting in improved fuel consumption.
(b) Reduced number of interconnections hence improving reliability.
How can I find the CAN Bus Wires?
It is easy to identify the wiring as it is just a twisted pair of thin wires. These will be found all over the vehicle - sometimes in with a conventional main loom. The wires are called 'CAN High' and 'CAN Low' and it is important that any interface is connected correctly to these wires.
So much for progress. :(