check engine light on - what could it be? on 1991 Mercury Topaz

just had oil changed and I know I need to get a tune up - could it be spark plugs/fuel/fuel filter/sensors?

Asked by for the 1991 Mercury Topaz
What are the trouble codes?
Unfortunately there are none-just the light. BTW - sorry for not responding as quick as you did! I am thinking it just needs a tune up - I got this car from my grandmother and I don't know the last time it had a tune up.