The check engine liight usually comes on because the O2 sensor isn't responding correctly to mixture adjustements (that is, give a lean signal no matter how rich the mixture is supposed to be, or vice versa), and the computer turns the light on.

You need to have the codes read, just fishing about will cost you a bunch of money.

There are a number of things that will cause the light to come on, reading the codes will give you a place to start.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!