Afaik there are lots of "deletes" that get done on diesels that result in the need to have the check engine light deactivated, so even if you get the light working you will then probably then have a lot of expense in adding the deleted systems back in.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)