Service engine soon light

[chevymotocross]

I recently bought an 87 silverado 2wd with the 305 fuel injected engine. It just recently passed emissions for me, and the previous owner had new piping, mufflers, and cats from manifolds back (dual exhaust). I took it on a pretty long trip today, on the highway the service engine soon light came on. Truck still ran great and all the gauges did not hint a problem. I opened the hood and everything was fine, including fluids. I let the truck sit for a couple hours when I reached my destination. When I started it up, the light was off. On my way home, about 30 min. into my trip, the service light came on again. What is going on here? I guess I could have someone hook up an OBD computer and see but if the light is off, will it still read the errors or does the light have to be on? Also, before my trip, I filled up with 87 octane fuel and added SNAP Fuel Injection treatment since it does run a bit rough cold.