Often emissions test for CO+CO2 as a measure of combustion efficiency. An ideal burning of a hydrocarbon would only release water vapor and carbon dioxide. Unfortunately engines are far from ideal, and the real chemistry of combustion is very complicated as a result. The pollutants in exhaust are what we call the byproducts of combustion.
Bad fuel air ratios that cause noxious emissions are caused by the engine operating outside of its ideal rpm range. Engines have only one speed at which they produce the best ratio of power output to low emissions. Thus STAR smog check station is ready for cooperation and assistance.

Unfortunately to work with transmissions for variable vehicle speed and power output variation, engines must operate over a wide range of speeds from idle to redline.
Incomplete combustion also causes particulate matter and VOC/ HC to form. These dust/ soot/ vapors are extremely toxic: they are the parts that give some of the color and most of the nasty smell to vehicles exhaust and smog.
Toxic vehicle emissions are especially notable in older vehicles with inefficient engines, but you can smell them in the exhaust of newer vehicles too; they are most pungent when the engine is cold and first starting up after having cooled off completely. Warm engines burn more cleanly than cold engines. Using a block heater for example to preheat the engine can dramatically reduce cold start emissions, especially in very cold climates. STAR smog check station, equipped with the latest testing devices, are ready to check your vehicle

At idle an engine produces excess VOC/ HC because the engine cannot burn its fuel completely at low speeds. These emissions react with sunlight (photo-chemical smog) forming hundreds of compounds in smog, and contributing greatly to ground level or tropospheric “bad” Ozone.
Traffic congestions that causes stop and go traffic results in a lot of engine idling, and thus increases the amount of HC/ VOC in the air hanging around the traffic corridor. You can see and smell the plumes of smog forming over interstate 5 in California when stop and go traffic is combined with low wind conditions and warm temperatures. You can almost always “Smell” stop and go traffic congestion.
SOX’s, NOX’s, CO and VOC’s / HC’s in vehicle emissions are the chemicals responsible for smog formation. All engines that “burn” a fuel release smog forming emissions.
Vehicles that burn oil or petroleum products and distillates (gasoline, diesel, natural gas, propane or another hydrocarbon) all suffer from incomplete and non-ideal combustion that produces nasty smelly and toxic smog forming emissions.  Have you ever asked yourself: “where is the closest station for STAR smog check station?”