What year is your car?
If a 1989 to 1993, it'll have the Gen1 VMM.
For those, there's a reset button to the left of the display. You have either a button head, or a hole.
If the hole, you need to use a small "pusher" like a paper clip that's been straightened out.
In either case, be pressing the reset button as you turn the car on (don't even have to start it). The three lights below the "Oil" light will blink, then it will light up the "OK".
For a 1994-1995, I dunno offhand.
RwP