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What is an emissions test?
Every internal combustion engine in a car, truck, or SUV gives off exhaust gases as a byproduct of the combustion process. Some of those gases are harmful to humans and to the environment. An emissions test (sometimes referred to as a “smog test”) is a set of procedures aimed at making sure your vehicle is not emitting pollutants that exceed the legal requirements of the state in which you live.
Why is an emissions test important?
The exhaust that comes from the tailpipe on your vehicle has the potential to impact the quality of the air you breathe. That is especially true if you live in a densely populated area, such as in a large city. Carbon monoxide (CO) is an invisible gas that results from incomplete combustion. CO is extremely toxic to humans. Most modern engines curb the amount of CO that is emitted, but if your engine is not running properly (or if it is in an older vehicle), CO levels might be higher than normal.
Benzene is another ingredient in the automotive exhaust cocktail. Benzene is a known carcinogenic, and high levels of inhalation can significantly impair your health. And hydrocarbons that escape from the tailpipe or evaporate are also detrimental to your wellbeing.
Health is not the only victim of pollution. Greenhouse gases and other compounds that come from your engine can also wreak havoc on the environment. Carbon dioxide (CO2) has long been thought to be a major contributor to climate change. Nitrogen oxides are highly reactive and contribute to smog. Sulfur dioxide forms acids when burned and can lead not only to smog but engine corrosion.
Beyond the health and environmental implications of emissions testing, the process can also help to identify potential problems with your engine. An excess of pollutants can alert a technician to needed maintenance that can prevent mechanical failure in the future.
What vehicles require emissions testing?
In an effort to reduce air pollution, the Federal Clean Air Act was amended in 1990. In response to the revised legislation, the Environmental Protection Agency (EPA) designed a set of emissions standards that could minimize the volume of hazardous pollutants expelled by motor vehicles. As a result, many states in the US adopted testing requirements and protocols for vehicles registered there.
Today, approximately thirty states require some level of emissions testing by law in order to register a vehicle. The degree of testing varies by state, and in some cases, by county.
While some states do not require emissions testing at all, the states that do offer certain exemptions from testing. For instance, any vehicle that does not produce greenhouse gases – including fully electric cars and hydrogen fuel cell cars – do not need to be tested. Brand new vehicles are often exempt. So might very old vehicles. Again, it depends on your jurisdiction.
Check with your local DMV to determine if your vehicle needs to undergo an emissions test.
How is an emissions test performed?
The procedures for testing emissions might differ from one locale to the next. In general, a technician will perform two tests on vehicles produced since the mid-nineties.
First, a diagnostic scan tool is connected to the vehicle’s data port to access the engine control module (ECM), the computer that controls and monitors engine functions. The technician will check for evaporative emissions and other forms of pollutants recorded by the computer. If all is performing as it should, a special sensor will be inserted into the tailpipe so that the exhaust gas can be analyzed. The sensor will test for CO, CO2, hydrocarbon, and oxygen. Some will also check for nitrogen oxide. Diesel vehicles will be checked for soot as well.
How long does an emissions test take? The procedure takes anywhere from fifteen to thirty minutes and is required as a routine part of the vehicle registration renewal process annually or every couple of years. If your vehicle passes inspection, you will receive the results immediately and your vehicle can be registered at the DMV.
What happens if you fail an emissions test?
If, on the other hand, your vehicle fails the test, you cannot drive the vehicle or renew the registration until you have completed repairs to remediate the problem. In some cases, you might have a grace period in which you can seek repairs. But outside of that grace period, your vehicle cannot be legally driven.
If your vehicle does not pass the test, make sure to take it to a trusted repair shop. Give them test results so that the technician can understand the nature of the trouble and offer you repair options.
What can cause a failed test?
There are a number of potential problems that can lead to a failed emissions test. Issues with various vehicle systems can have an impact on the exhaust system and level of pollutants expelled from the exhaust.
Not all of the reasons for a failed test are directly related to the test itself, however. For instance, if your battery was recently disconnected, the on-board diagnostic system might not have enough information stored in memory to satisfy the testing protocols. You might need to drive the vehicle longer in order for more data to accrue.
Issues with the Data Link Connector (the connection to the diagnostic system) can cause a test failure as well. So can the presence of a check engine light.
As for actual test failure issues, the most common stem from malfunctions with the following:
- Exhaust system components, such as the catalytic converter
- Fuel delivery system components, such as the fuel injectors
- Ignition system components, such as the spark plugs or ignition coils
- The air injection system
- Sensors, such as an oxygen sensor
- Faulty or loose gas cap
- Poor vehicle maintenance, such as infrequent oil changes or engine air filter changes
What happens if you fail the test twice?
If your vehicle happens to fail an emissions test more than once, perhaps due to incomplete repairs or multiple problems, you might have a few options depending on where you live.
In some cases, you might be able to apply for an extension or a waiver if you can demonstrate that, due to an economic hardship, you are unable to afford the necessary repairs. You might be able to get a waiver if you spent a certain amount of money on repairs to address a problem that led to a failed emissions test and yet did not resolve the problem. And there might be a waiver if the cost of repairs exceeds a threshold for the year, make, and model of your vehicle.
How do you fix an emissions test failure?
If your vehicle fails an emissions test, the remedy depends on what caused the fault in the first place. Included below are several examples of repair procedures aimed at the kinds of problems that can lead to a failed emissions test.
In any case, no matter what the cause, you must have your vehicle reinspected once repairs are complete in order to pass the test and receive the go-ahead to renew your vehicle registration.
Repair procedures related to emissions test failure:
- Oil change
- Gas cap replacement
- Oxygen sensor replacement
- Fuel injector replacement
- Spark plug replacement
- Spark plug wire replacement
- Ignition coil replacement
- Engine air filter replacement
- Catalytic converter replacement
- PCV valve replacement
Failed Emissions Test FAQs
What is an emissions test?
An emissions test is a set of diagnostic procedures aimed at checking the levels of harmful pollutants emitted through the exhaust system on a vehicle.
Why is an emissions test important?
An emissions test is an important step toward limiting the levels of pollutants that can be harmful to humans and to the environment. A majority of states in the US have adopted standards for testing vehicles according to EPA standards.
What happens if you fail an emissions test?
If your vehicle fails to pass an emissions test in a state that requires a positive result for renewal of registration, you will be unable to legally drive your vehicle until repairs have been completed and it passes a subsequent test. In some cases, you might be eligible for an extension or waiver.
Can you pass an emissions test with an oil leak?
A major oil leak can result in elevated levels of hydrocarbons that might keep your engine from passing an emissions test. A small leak, however, should not impact the test results.
What year vehicles need an emissions test?
Which vehicles require emissions testing depends on the particular state in which you live. In general, new vehicles (sometimes classified as those less than three years old) are exempt. In some cases, older classic or historic vehicles are also exempt from the test.
How do you fix an emission test failure?
The repairs required to fix an emissions test failure depend on the cause of the negative result. For instance, if your vehicle fails due to elevated hydrocarbons, you might only need an oil change. If it is due to fuel vapor loss, you might need to tighten the gas cap. But some problems require more expensive repairs, such as replacement of the fuel injectors, an oxygen sensor, or even the catalytic converter.
