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What’s in an Emissions Test, and How Do You Pass?

To reduce air pollution, most states require drivers to have their vehicles tested to identify any cars that produce excessive emissions. Failing a smog test can prevent you from properly registering your vehicle, which could result in a fine or ticket. How can you make sure that your vehicle will pass?

States have different requirements for smog testing, and their terms and exemptions can vary. For example, in California, most vehicles over six years old must be tested every two years, but gas-powered cars built before 1975 are exempt from testing. In New York, tests are conducted annually. 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.

Many vehicles fail smog tests. According to the Sacramento Bee, more than 10 percent of 11 million California vehicles tested in 2015 failed their smog test. And older vehicles are more likely to fail than newer ones.

With proper preparation, you can improve your car’s chances of passing a smog test. These are some useful steps you can take:

6 Steps to Help Your Car Pass an Emissions Test

1. Give your car some time to warm up

According to the experts at Smog Star Express, a California-based smog test facility, warmer engines have a better chance of passing a smog test. With this in mind, it’s a good idea to run errands before taking your car in to be tested. This will ensure that the engine is primed for the test.

2. Make sure your car’s maintenance is up to date

This will optimize your vehicle’s performance and improve your chances of passing the test. For example, if you haven’t gotten an oil change within the recommended interval, your car’s oil may be contaminated, hurting your car’s performance on the test. Carfax’s Car Care app can help you keep track of your car’s maintenance.

3. Put some miles on the car if you’ve replaced your battery recently or had maintenance done

However, it’s important to ensure you put at least 100 to 200 miles on your car between maintenance work and a smog test.

In many cases, mechanics performing a 10-, 15- or 20-point check during an oil change or other maintenance work will disconnect your car’s battery to facilitate this process.

Disconnecting the battery can temporarily erase your car’s memory chip, which needs to be populated for your vehicle to pass a smog test. It can take up to 200 miles of driving for the chip to be ready. If you’ve recently charged or replaced your car’s battery, put up to 200 miles on it before taking it in for a smog test.

4. Make sure your car’s “check engine” light isn’t illuminated

Sometimes your car’s “check engine” light can illuminate for minor reasons, and it’s easy to delay taking the car in to have the problem investigated. However, if you’re taking a smog test, knowing that an illuminated “check engine” light is automatic grounds for failing the test is essential.

This light can be triggered by a faulty oxygen sensor or catalytic converter or by a gas cap that is loose, damaged, or missing. This problem can also be linked to a damaged air-flow sensor or worn spark plugs.

If your car’s “check engine” light is on, resolve the problem before attempting a smog test.

5. Make sure your tires are properly inflated

As part of the testing process, some states require cars to be driven on a piece of equipment known as a dynamometer. If the car’s tires are properly inflated, it will have greater stability on the machine, improving your odds of passing the smog test.

Use a gauge to check the air pressure of your car’s tires and compare this pressure against the manufacturer’s recommendations in your owner’s manual or on the sticker on the driver’s door jamb. If the tires are underinflated, increase the pressure before taking your smog test.

6. Consider a fuel additive

Smog tests measure emission levels, and fuel additives have been shown to lower emissions. These additives can usually be added to the gas tank when refueling your car. They remove carbon deposits lodged within the engine. In doing so, they boost performance and lower emission levels.

Dyno testing
Photo Credit: Getty / shaunl

What Is an Emissions Test?

In general, there are two actual emissions inspections, an OBD test, which uses your car’s computer to check for issues, and an exhaust gas analysis, which analyzes the exhaust directly.

What is an Onboard Diagnostics (OBD) Emissions Inspection?

An OBD emissions inspection checks that the vehicle is running to its standards by tapping into its computer system. In the OBD test, if any monitors are incomplete – because the car’s memory chip has not yet repopulated its essential information after the battery was disconnected – or failed, the vehicle will not pass the smog test. Incomplete monitors usually require the car to be driven more, but failed monitors must be diagnosed, repaired, and retested.

What is an Exhaust Gas Analysis?

The OBD test can cover basic emissions, but the exhaust gas analysis inspection is more precise. Emissions are measured and compared using an exhaust gas analyzer in the tailpipe. 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.

In the exhaust gas analysis, the engine must be diagnosed, repaired, and retested if any gases exceed the limits.

Why Do Cars Fail Emissions Tests?

There are several common reasons that vehicles can fail an emissions inspection:

  • Fuel trim problems: This is usually caused by faulty oxygen sensors or malfunctioning air metering, which can result in increased fuel consumption and emissions. In some cases, this can lead to misfiring or catalytic converter meltdown.
  • Evaporative (EVAP) emission problems: Loose, incorrect, or missing gas caps are the No. 1 cause of 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.
  • Catalytic converter failure: We touched on this already, but catalytic converter failure is often related to poor maintenance and 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 engine problems.
  • Weak or missing spark: Weak ignition coils, worn spark plugs, and poor spark plug wire insulation can all lead to weak spark or inconsistent firing. This, in turn, leads to poor combustion and engine misfires, which increase fuel consumption and hydrocarbon emissions. It can damage the catalytic converter as well.
  • Temperature problems: 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.

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Benjamin Jerew contributed to this report.

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