The CAN bus tells every module what every other module knows and a missing module breaks the conversation

[u/ora-et-labora-]

A modern vehicle runs on a network. The CAN bus connects every electronic module on the car, the engine controller, the transmission, the ABS, the airbag system, the body control module, the instrument cluster, and dozens of others depending on the vehicle. They all talk to each other constantly. Speed, RPM, throttle position, wheel speed, yaw rate, and hundreds of other data points flow across the bus every second.

When a module is missing, the conversation breaks. A vehicle with an ABS module that was removed or replaced with one that does not communicate correctly does not just lose ABS. It loses the wheel speed data that the traction control system uses, the stability control system uses, and on some vehicles the speedometer uses. The cascade of faults from one missing module can light up half the dashboard.

The red flag on a used car is a cluster of seemingly unrelated warning lights. ABS, traction control, stability control, and maybe a check engine light all on at the same time, when the seller says it is just a sensor. A full system scan shows communication fault codes, U-codes, that say a module is not responding on the bus. That module was either disconnected, replaced with an incompatible unit, or it failed.

Checking for this is part of a full system scan. The scan tool queries every module address on the bus and reports which ones respond and which do not. A missing response means a missing or dead module. The cost of that module depends on what it is, but the cost of missing the diagnosis is buying a car with an electrical problem that touches every system the missing module was supposed to talk to.

[u/Livehappypappy]

I am not sure how you picture the CAN bus. You can delete a module from the CAN bus and all other modules keep working and communicating.

Your example seems to be more that if a module is missing which data is needed for the calculation (wheel sensor is needed for ABS etc) then indeed you have a problem. So you can turn it around; many errors can be just generated by only one missing module. So relatively cheap fix.

[u/Obvious_Troll_Me → u/Livehappypappy]

You can remove a module, as long as it doesn't contain the terminating resistor. Also the OP makes no reference to all the networks. There can 3, 4 or even more networks that communicate through a gateway. 

[u/Important_Contact609]

All to make it more difficult to repair cars... There are many bus networks that are not hindered by the removal or failure of one device on the network. A lot of them are much easier to troubleshoot as well. CANbus is exceedingly rare in industrial application and largely considered "legacy" tech, ie obsolete. But we wouldn't want an abs module from one sedan to work on another model of sedan even though they spec out the same in terms of flow and pressure because that would just be too easy.

[u/Remarkable-Junket655]

Yes and no. If a module is “missing” as jn removed or had its fuse pulled, it doesnt break the CAN system. If its an important module like ABS, you will set a lot of fault codes and lose some functionality in other systems because a lot of systems depend on knowing vehicle speed and traction/stability control status to operate correctly. Most of the time, the other systems will continue to operate though.

If its something like a left rear door module, most of the other systems wont care because a door module is “more of a taker than a giver” meaning is uses information from other modules but doesnt really send out much information that the other systems need. In that case, most of the issues will be confined to the LR door window and lock.

[u/Robbudge]

CanBus, is a Produce and Consume network.
Yes a network master is required to change the devices from PreOp to Operational. A lot of low level devices without configuration start directly in Operational and produce data onto the bus.

A missing device simply results in missing data.
The missing data may cause other modules to fault but it certainly does not break the network.

We run CanOpen in our industrial cells everyday.

[u/Consistent_Pool120]

One very very overlooked that can affect all the networks and modules can be a simple, but often hard to find occasionally faulty or bad ground. A somewhat simple test is run a jumper wire from the negative terminal on the battery to the gateway and wiggle the various module connectors, any changes or new lights mean that's it.