additional functions of the self diagnostic systems.

i was talking abt self diagnostics in my previous post, now i shall continue after reading up.
ok, once the ECM has recognised the fault only then will it be able to implement other functions, some common functions are performed by the ECM whenever a fault is recognised. the ECM can display the malfunction indicator light on the dashboard. thus indicating to the driver that there is a fault or problem with their own vehicle. also, a system failure can cause the engine to run poorly and also to generate high emissions. under these conditions the ECM may be able to control the engine management system to start operating in a" limp home" system, something like a fail safe mode. when the ECM implements the fail safe mode it normally substitutes a pre-set value for the failed sensor which ususally ensures reasonable engine operation. for some older cars, from what i know, the ECM can provide some form of coded output, whcih enables the fault code to be accessed to be recognised or retrieved by some form of code reader such as a blink code transmitted via the dash warning light or led.
i am not going to talk abt how the systems recognises faults, because it will be different for every cars systems, however though i am going to touch on how the systems recognises faults. then son on explaining the difference between the two type sensors like the voltage generating and reference sensors, hmm...is that the word it.