How Car Computers Work

­­Another benefit of having a communications bus is that each module can communicate faults to a central module, which stores the faults and can communicate them to an off-board diagnostic tool.

This can make it easier for technicians to diagnose problems with the car, especially intermittent problems, which are notorious for disappearing as soon as you bring the car in for repairs.

BATauto.com: Technical Info Pages lists the fault codes stored in the ECU for various carmakers. Sometimes, the codes can be accessed without a diagnostic tool. For instance, on some cars, by jumping two of the pins in the diagnostic connecter and then turning the ignition key to run, the "check engine" light will flash a certain pattern to indicate the number of the fault code stored in the ECU.

Let's take a look at how microprocessors and communications standards have made cars easier to build.