2001 Nissan Maxima "Service Engine Soon" Warning Light?

I have a 2001 Nissan Maxima and to correlate with another question that I posted, I was wondering about the "Service Engine Soon" warning light that is on my dashboard. I went to go get it checked out and there was nothing wrong and the light went off. It then came back on and now refuses to go off. Any suggestions on ways to fix it? Thanks!