1995 Toyota Tercel check engine light

1995 Toyota Tercel mileage: 91,000. My problem concerns my 1995 Toyota Tercel, 91,000 miles, 4-speed manual transmission, most recent oil change was about 1800 miles ago, and no major repairs since purchasing the car at 40,000 miles. I recently made a 1600-mile trip from Virginia to Colorado; I had the oil changed, fluids topped-off, and air filter replaced. Additionally, I had an emissions inspection done a few weeks before, and the results were excellent. Approximately half way through the trip and some very bumpy interstate driving in Arkansas and Oklahoma, my "Engine" light came on. I pulled over immediately to check under the engine, checked the oil level, belts, hoses, etc. And found no noticeable irregularities, except that the overflow container for the radiator fluid was empty. There was still plenty of fluid in the radiator itself, but I replaced the fluid/water mixture anyway. I noticed no odd smells or drivability difficulties, so I continued driving. I stopped overnight and the following morning, I checked my owner’s manual and it said that this light comes on due to a problem with the engine's electrical system. Being that the roads were rough, I suspected maybe a fuse had been knocked out, so I removed, inspected, then replaced all of the fuses. I started the car and the engine light was off. I continued driving, but after about 6 hours of interstate driving (70-80 mph) the light came on again. Still, I did not notice any smells, sounds, or drivability problems. I continued to drive as I was in the middle of nowhere and arrived in Colorado, still with the engine light on. I spoke with a technician about this before and his recommendation was that the oxygen sensor could be the problem. I have researched the function and possible problems of the O2 sensor and it seems like if that were the culprit, my exhaust would smell like "rotten eggs" OR produce a black smoke (depending on lean or rich mixture). Is this correct? Or could the sensor be malfunctioning WITHOUT a real problem with the mixture? What type of problems could develop with continued driving? Could there be another cause to the engine light coming on? Is there some way to check the sensor myself without having to take to a mechanic? Could the missing radiator fluid have caused something to go wrong (I have noticed no leaks in the overflow tank since)?Could the change in altitude cause something? I apologize for the length of this, but wanted to make sure to get all of the information here. Thank you in advance for your help!