The Mil is set after one or more monitors have failed. For a number of non
critical faults, the monitor must fail several times.
The same is true for resetting the MIL, if the failing test succeeds several times,
the system assumes you may have cured the condition. Example: the MIL is set
when the leak test of the fuel system fails, which can be caused by the filler
cap not being closed properly. This monitor runs once every time the car is
started (and a number of conditions are met).
If, after filling up, you were to close the filler cap as it should, the MIL will
eventually be reset. This scenario may happen without the driver realizes what
exactly is going on.
I think the dealer's guess about the cause is just that, a (wild) guess.
Although action is required whenever the MIL is set, you shouldn't feel
threatened when it lights, it is a warning.
If a critical condition arises, the general warning light (triangle) will light red,
whereas the MIL (yellow) indicates problems concerning the emission sytem.
Willy
144 GL (1974)--->244 GL (1982)--->940 GLE 2.3i (1992)--->XC70 2.5T (2004)--->XC90 T5 (2018)
Bookmarks