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That is a really good question. I wondered that also the first time I say this fault with a egr cooler that was plugged. Long story short - the computer in those years knows there is fault but doesn't actually know high from low. There are other instances of this on this year cars with boost pressure upper limit exceeded and the turbo will be blown up and not boosting at all. The computer is confused. The next part of your question I will answer also - The systems are constantly monitored but some systems require more than 1 or 2 trip cycles to set a fault or a check engine light. Some faults are 1 trip cycle and others are 2 or 3. Trip cycle is defined as a start up and drive to operating temperature and then cooled off again = 1 trip cycle. hope this helps let me know if you need anything else.