My '01 X5 has just thrown the Service Engine Soon, yellow light message.

What confuses me is that the service interval indicator still shows 4 green lights. The oil is fresh and clean.
(The only other indication of needing service is the form letter I got from the dealership saying they thought it was about time for me to come in and contribute finacially to the fund.... coincidence? I think not...) but I digress...

any thoughts? I have consulted the owner's manual and maintenance manual as well as the separate hand out on service intervals... no info unless I over looked it.
THX