If your vehicle fails, it will need to be repaired and brought back for reinspection. Motorists are provided with information to help the repair technician diagnose the cause of the failure.   

A Word About Repairs
Although motorists are free to choose any repair shop, we suggest you consider having your vehicle repaired by a qualified technician who is trained and experienced in the repair of emissions components. A recognized repair facility employs at least one technician with ASEL1, WISETECH or other equivalent emissions training.  Only repairs performed at a recognized repair facility will be applied to cost waivers.  For more information on cost waivers, click here.

The repair of emissions related failures has emerged as a specialty in automotive repair. Just as all repair facilities do not work on transmissions or do front-end alignments, not all repair facilities work on emission failures. The diagnosis and repair of vehicles that fail an emissions or On-Board Diagnostic inspection can involve repair or replacement of multiple systems and components and can be very challenging to even veteran technicians. Statistics show that the average vehicle owner is unable to properly diagnose and repair emission failures because of the specialized training and experience required to repair these vehicles.

For questions regarding the Wisconsin repair/retest process, you may call the program hotline at 1-866 – OBD – TEST.

Emission-Related Recall and Voluntary Service Campaigns
To find out if your vehicle is on the EPA’s Annual Summary of Emission-Related Recall and Voluntary Service Campaigns Performed on Light-Duty Vehicles and Light-Duty Trucks, please visit www.wisconsinvip.org, www.epa.gov/otaq/recall.htm, or call 866-OBD-TEST (866-623-8378). The Annual Summary of Emission-Related Recall and Voluntary Service Campaigns Performed on Light-Duty Vehicles and Light-Duty Trucks can also be printed from the analyzer in the Print/View documents area in the Utilities menu by a certified emissions inspector.

Vehicle Emissions - What They Are and How They're Formed
Air pollution in the United States comes from many types of engines, industries, and commercial operations. Pollution sources that move, such as autos, trucks, snowblowers, bulldozers, and trains, are known as "mobile sources." Examples of all other (non-mobile) sources of air pollution include power plants, factories, and manufacturing processes.

Mobile sources pollute the air through combustion and fuel evaporation. Motor vehicles and equipment typically burn fuel in an engine to create power. Gasoline and diesel fuels are mixtures of hydrocarbons, which are compounds that contain hydrogen and carbon atoms. In "perfect" combustion, oxygen in the air would combine with all the hydrogen in the fuel to form water and with all the carbon in the fuel to form carbon dioxide. Nitrogen in the air would remain unaffected. In reality, the combustion process is not "perfect," and engines emit several types of pollutants as combustion byproducts.

These mobile source emissions contribute greatly to air pollution nationwide and are the primary cause of air pollution in many urban areas. Learn more about how mobile sources contribute to four significant air pollutants and how these pollutants affect human health and the environment in the following pages:

Mobile sources also produce several other important air pollutants, such as particulate matter, air toxics and greenhouse gases. Nationwide, mobile sources represent the largest contributor to air toxics. Air toxics are pollutants known or suspected to cause cancer or other serious health or environmental effects. Greenhouse gases, such as carbon dioxide (CO2), trap heat in the Earth's atmosphere, contributing to global climate change.

The three emissions that are measured for a pass/fail decision in the emissions test are carbon monoxide, hydrocarbons and nitrogen oxides. The following is a brief description of the gases and how they are formed.

Carbon Monoxide (CO)
Carbon monoxide is harmful because it reduces oxygen delivery to the body's organs and tissues. It is most harmful to those who suffer from heart and respiratory disease. High carbon monoxide pollution levels also affect healthy people. Symptoms may include visual impairment, headache, and reduced work capacity. Unlike many other air pollutants, carbon monoxide levels in the outside air typically peak during colder months

Carbon monoxide is a colorless, odorless gas that forms when carbon in fuel is not burned completely. Carbon monoxide is a component of exhaust from motor vehicles and engines. Carbon monoxide emissions increase when conditions are poor for combustion; thus, the highest carbon monoxide levels tend to occur when the weather is very cold or at high elevations where there is less oxygen in the air to burn the fuel.

Carbon monoxide emissions are the result of an incorrect air/fuel mixture. In a carbureted vehicle, improperly working floats, power circuit and choke circuit could cause high CO. In fuel injected vehicles, improperly working injectors, pressure regulators, coolant sensors or MAP/MAF sensors could all contribute to high CO emissions. Some other common systems or components related to CO emissions are the air filter, PCV system, canister purge, O2 sensor and air Injection system. Due to the complexity of the internal combustion engines, there may be layered problems related to high CO emissions.

Hydrocarbons (HC)
Hydrocarbons, which may take the form of gases, tiny particles, or droplets, come from a great variety of industrial and natural processes. In typical urban areas, a very significant fraction comes from cars, buses, trucks, and nonroad mobile sources such as construction vehicles and boats.

Most motor vehicles and engines are powered by hydrocarbon-based fuels such as gasoline and diesel. Hydrocarbon pollution results when unburned or partially burned fuel is emitted from the engine as exhaust, and also when fuel evaporates directly into the atmosphere. Hydrocarbons include many toxic compounds that cause cancer and other adverse health effects. Hydrocarbons also react with nitrogen oxides in the presence of sunlight to form ozone.

Some common causes of high hydrocarbon emissions include improper ignition timing, vacuum leaks, engine mechanical problems and malfunction of components related to the ignition system, such as spark plugs, secondary wiring, distributor (if equipped) and coil. Due to the complexity of the internal combustion engine,  other components may cause high HC as well.

Nitrogen Oxides (NOx)
Nitrogen Oxides is a group of highly reactive gases that contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. One such gas, nitrogen dioxide (NO2) can often be seen combined with airborne particles as a reddish-brown layer over many urban areas. Nitrogen oxides are formed when the oxygen and nitrogen in the air react with each other during combustion. The formation of nitrogen oxides is favored by high temperatures and excess oxygen (more than is needed to burn the fuel). The primary sources of nitrogen oxides are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.

Nitrogen Oxides in vehicles are caused by excessive combustion chamber temperatures. Some of the common causes of high NOx emissions are problems with the vehicle's Exhaust Gas Recirculation System (EGR), improper ignition timing, lean air/fuel mixture and malfunctions in systems that control engine temperature, such as the thermostat and cooling fan, and vacuum leaks. Due to the complexity of the internal combustion engine, other components may cause high NOx as well.