Well, there IS a point where the computer says, "All I know, is that something is screwed up purty bad, and I can't deal with it anymore"......

 

Yes there is, as explained.
Missfire detection, oxygen levels out of range by the ox sensor, test routines controlled by the executive program controller for all full time testing.
The reporting of faults is usually on at least 3 levels.
1. immediate.
2. short time threshold out of range.
3. long term thresholds out of range.

With these ways of reporting faults,
an electrical fault us usually an immediate reporting action,
a fault that has an effect on emmisions is usually buffered to some degree to make sure it is not a false or temporary condition brought on by conditions,
and longer term shifts in operation away from expected limits set by software.
Tables are the usual reference with limits at each end to trigger a code and warning lamp.
Then there are Ques that accumulate data to a limit before setting a code and lamp. These are also limit and time tables to keep false reporting to a min.
It depends on what the fault is as to the effects on drivability.
Once you get into the habit of thinking about all this in digital and statisital terms, then things begin to come much easier.

The 401 code gives people a lot of hassle when it really is not a big mistery.
First, the EGR regular operation "has" to pass dignostics first for the flow to be tested and have any chance of passing.

That being said the 401 code tells you the EGR and it's EVR is not the first line of concern.
What is DPFE? The term gives many hints. It is a measuring device for looking at the differences in exhaust pressure using two ports. To do this the ports need a restriction to measure accross as well as a programmed flow rate to measure against for expected results.
All the actions to bring this about are written in software and use the hardware to accomplish it and use the DPFE to measure and report the results. Any failure either hardware or software will break the operation.
Well this all can go on as a book. Anyone interested has to make the efforts to learn, then you can't get it all.
Base line is that we are running cars and trucks that are fully computer controlled. Those that just try to repair on a first time or once in awhile are hard put to be successful unless they have a lot of luck.
Not much different than when your PC crashes, gets taken down by virus etc and all the hassle it take to get it all back up. Been thru it several times over the last 15 years. After awhile you get to be pretty good at it. 8 discs to reboot the computer, hard disk copy programs, re-install all the support programs, the ISP re install.
It's all fun and games.