My (25 year old) gas cap's o-ring inside the car had tiny spider cracks (very difficult to see unless the o-ring is dry). I replaced the cap with a new one and cleaned the metal filler neck's "lip" .....that took care of the CEL.
Just a suggestion....even though you say the cap seals "OK" a new cap is very cheap....so it might be worth trying anyway. That, along with cleaning the filler neck's lip that contacts the cap's o-ring is worth a try.....clear the code and drive for a few days and see if the CEL stays off.
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98' Boxster
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