engine light

hey group, im hoping someone has some insight to my problem with my 91 sentra. I am overdue on my registration because I need a smog check,however, my engine light stays on althought there is nothing wrong with my car. it does go off at times but im never near the smog station when it does.I m trying to avoid a big fee in getting it checked out when it possibly could be somethin as simple as tightening a screw underneath my car .Im hoping someone has some insight on this problem.I need help asap thanks for listening .I am in the bay area california ,san jose if anyone is from here and knows a good reliable trustworthy mechanic