1995 Toyota Corolla Engine LIght

My engine light came on for the first time about 2 months ago. It came on right after I took a drive in the snow. There was ice and snow on the roads so maybe I was overworking it. My car was also overdue for an oil change. I took it to the dealership and they said it needed a modulator and another part to go with it. It had something to do with the valves on the engine. My car wasn't running any differently though. I had my oil changed the next day and the day after that. The light went off. I didn't go back to get the modulator. This morning as I am driving to work, the light comes on again and stays on for about 30 minutes and goes off! What the heck does this mean? Is a modulator necessary? I am due for another oil change, could that be why it came on and went back off?