Nissan Motor Co.'s North American unit said it is in the final process of certifying in California a gasoline-fueled, "super ultra-low emission vehicle" for the 2000 model year. Nissan said the 1.8 liter version of its 2000 Sentra compact sedan is expected to emit about one-fourth of the unburned hydrocarbon and one-tenth of the oxides of nitrogen emissions of the cleanest gasoline-powered car sold now in California.