Hoping somebody has the answer....1991 945 SE
Service Engine light illuminates on start up every time and goes out after approximately 2 minutes.
No noticeable change in performance or running. Any thoughts/suggestions?

It's a "service reminder" light. Take out the small rubber cap (if it's still there) on the face of the instrument panel, and depress the plastic plunger while the light is on using a thin but not too sharp a tool, like a ball point pen. This resets the light for 5,000 miles. Replace the cap.

Excellent!!!
Thanks guys. Followed Ted's advice. Removed the rubber plug on the face of the instrument panel and inserted a dull ice pick like device with a slight amount of pressure while the light was illuminated....bingo, no more service reminder.

Thanks again!

Don't worry about it. It is basically an oil change reminder. Volvo probably programmed it to go off every 5000 miles or so. All Volvo's of that era had the same thing. Both my 90 240 and 95 850 have it. There is a way to shut it off, but I don't know how on a 940...