Typically, where there’s smoke there’s fire, but though no “smoke” can be seen from most automobile tailpipes, there certainly is a fire inside the engine. For most of automobile history, extracting power was the most important consideration in engine design and troubleshooting. Much of that attitude, however, has changed since the discovery that exhaust emissions and evaporative emissions are responsible for several problems, such as health problems, soil contamination, air pollution and climate change.
By the 1950s, people started to recognize the link between the proliferation of automobiles, declining public health and marred skylines. The Clean Air Act was passed in 1970, giving the Environmental Protection Agency the authority to regulate vehicle emissions through regulations, monitoring and testing. Since then, additions and amendments to the Clean Air Act have improved air quality and the quality of life for millions of people.
The EPA estimates that today’s vehicles — from cars and trucks to tractor-trailers and locomotives — are 99 percent cleaner than their model-year-1970 counterparts. The results are clear, as can be seen from this New York City skyline, comparing 1973 and 2013:
Federal and State Emissions Standards
While the EPA sets basic emissions standards for all vehicles sold in the United States, it’s up to each state to monitor emissions of vehicles sold in them. Federal emissions standards require equipment such as evaporative emissions recovery, containment, combustion, on-board diagnostics (OBD) systems, self-monitoring fuel trim and catalytic converters. For diesel engines, low-sulfur fuels are mandatory, as well as particulate filters and OBD systems.
States and counties must, at the very least, maintain federal emissions standards, but they may adopt stricter rules, too. California is well-known as setting the standard for itself — the car capital of the United States — as far back as the 1960s. Some basic California Air Resources Board emissions standards include tighter nitrogen oxide and hydrocarbon emission standards, and higher fuel economy guidelines, essentially tighter carbon dioxide rules. California’s fuel supply also is strictly monitored, containing less sulfur, reducing sulfur oxide emissions and benzene, reducing evaporative emissions.
About a dozen states have adopted California emissions standards, including Connecticut, Delaware, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington, and the District of Columbia.
Two Types of Emissions Tests
When it comes to meeting and maintaining emissions standards, different states and counties have determined when and how they monitor the situation. Several states specify no emissions inspection at all, while others require emissions testing or “smog check” when the vehicle is registered, transferred or every one or two years. Some states exempt certain vehicles, based on age or type. For example, some states exempt vehicles less than 5 model years old and more than 25 model years old. Within each state, some counties and cities specify their own testing standards.
When you register your vehicle, you’ll likely receive information regarding what emissions standards your vehicle needs to meet, as well as any applicable inspection schedule. In general, there are two actual emissions inspections, aside from generally checking that federal emissions equipment is intact or that no visible smoke is emitted from the exhaust system.
OBD or DLC Emissions Inspection
Aside from maintaining engine performance and reducing fuel consumption, one of the more important functions of the engine control module (ECM) is to monitor engine performance, which has a direct impact on emissions. The ECM continuously and periodically monitors the engine, transmission, fuel system and exhaust system. If a test doesn’t complete, its monitor will be set as “not ready” or “incomplete.” If a test completes, its monitor will set as “pass” or “fail.” The check engine light or malfunction indicator lamp will illuminate only if a monitor fails, but this is a good indication that the vehicle has an emissions problem. An OBD emissions inspection essentially checks that the vehicle is running to its own standards. A scan tool connects to DLC3 (Data Link Connector 3), on vehicle 1996 and newer, to check the ECM’s monitor status.
Exhaust Gas Analysis
While the OBD test can cover evaporative emissions and basic exhaust emissions, the exhaust gas analysis emissions inspection is more precise than the OBD test. Using an exhaust gas analyzer in the tailpipe, exhaust gases are measured and compared. Four-gas analyzers measure carbon monoxide), hydrocarbon, carbon dioxide and oxygen levels. Five-gas analyzers also measure nitrogen oxide emissions, and diesel exhaust gas analyzers measure particulate matter, or “soot” emissions.
Most Common Emissions Inspection Failures
In the OBD test, if any monitors are incomplete or failed, the vehicle will not pass emissions inspection. Incomplete monitors usually require the vehicle be driven a few more days to give them a chance to complete, but failed monitors will need to be diagnosed, repaired and retested. After exhaust gas analysis, if any gases exceed the limits, the engine will need to be diagnosed, repaired and retested. Here are some of the most common reasons vehicles fail emissions inspection:
- Fuel trim problems: Usually caused by faulty oxygen sensors or malfunctioning air metering, fuel trim problems can result in increased fuel consumption and emissions. In some cases, this can lead to misfiring or catalytic converter meltdown. Vacuum leaks can be hard to find, but they introduce unmetered air, leading to lean combustion and increased emissions.
- Evaporative emission problems: Loose, incorrect or missing gas caps are the No. 1 cause of evaporative (EVAP) emission problems. Some drivers refuel while the engine is running, and if the EVAP monitor runs, it’ll fail. Also, if the cap is loose, faulty or missing, the EVAP system will fail the leak test. The canister close valve and purge valve also are common failure points.
- Catalytic converter failure: We touched on this already, but catalytic converter failure most often is related to poor maintenance and poor driving habits. Cars need to be maintained and driven well, so keep up with regularly scheduled maintenance, drive on the highway at least once a week for half an hour and promptly repair any engine problems.
- Weak or missing spark: One key to good combustion is a strong and consistent spark. Weak ignition coils, worn spark plugs and poor spark plug wire insulation all can lead to weak spark or inconsistent firing. This, in turn, leads to poor combustion and engine misfires, which increase fuel consumption and hydrocarbon emissions, as well as damages the catalytic converter.
- Temperature problems: The engine should quickly reach and maintain a consistent operating temperature. Thermostat problems, radiator fan problems and extreme weather can lead to engine underheating or overheating, which can prevent the engine from running at its most efficient. This can lead to increased emissions, such as higher hydrocarbon for underheated engines, or higher nitrogen oxide for overheated engines.
Failing an emissions inspection isn’t the end of the world. Find a trusted technician to diagnose and repair the problem so you can have it retested. In some states, you may be able to get a waiver if you can prove you’ve spent a certain amount of money attempting to repair the problem. This article has been written as a general guide, but check with your local department of motor vehicles for specifics regarding emissions standards and emissions inspection requirements for you and your vehicle.