RE: 1991 190e check engine light
It could be nearly fifty different things. You will have to have the car tested. You could buy a book like the "E-class owners bible" 86-95 and bench-test "some" basic components yourself, if you are mechanically inclined, and up to the job. Sometimes it is a major component that you can bolt-on, but other times it be a fine-tune adjustment that requires the mother-ship etc.