Volvo: 1993 240 2.3 Volvo check engine light, warning light, board computer

Question
Engine was running rough and would not stay cranked when put in gear.  "Check Engine Light" came on.  Took car to mechanic and he replaced a spark plug.  Now the car is running fine but the "Check Engine Light" still stays on.  Is something else wrong?  Our mechanic advises to just reset warning light.  If he is correct, how can we acomplish this?  
the car is in excellent condition and only has 100194 miles.  I am the original owner.

Thanks for your help.

Answer
I'm not sure how to reset the warning light on a 93 model, but it seems to fall under the list of things your mechanic should perform when he fixed the car.  When he connects to your on-board computer to diagnose the problem, he should be able to reset it then.  If something else is wrong, the warning light will re trigger, so you don't have to worry about glossing over an additional problem.