Car Smog Test – What Is It and How to Pass the Emission Test?

In today’s world, as the number of motor vehicles on the road increases, the impact of exhaust emissions on the environment has also increased.

For this reason, the United States and European Union have passed legislation to set limits in order to control emissions. The Clean Air Act requested 33 states and local areas to conduct regular smog test for vehicle inspections. Most the localities have used OBD technology to inspects vehicle in use, which resulted in the practice of OBD Smog Test. The test is also known as smog check, I/M readiness testing or emission test.

According to the Environmental Protection Agency (EPA), typically a one passenger vehicle used daily equates to 4.6 metric tons of carbon dioxide annually.

So, what will help with this grand problem?

What is Smog Test?

Smog test, smog check, I/M readiness testing or emission test all refer to a vehicle testing system via an onboard technology to ensure that the vehicle in transport meets certain emission and engine standards.

Smog test is obligatory every 2 years in most of the states in the US. However, only some specific vehicles have to pass this emission test depending on their age, model and original registered area.

smog test: what it is
Smog check is obligatory in most of the states.

Why is Smog Test in system?

Firstly, the EPA has stepped in to ensure that all vehicles pass emission inspections. The test thus makes sure all vehicle meets national air quality standards.

Beginning in 1984, the United States played a leading role in recognizing and setting a limit on toxic emissions.  

As years passed, in order to ensure compliance with emission limits, vehicles began to be equipped with improved systems for self-diagnosis.

As a result, the OBD system or On-Board diagnostics was created. Later, OBD smog test or the application of OBD system checks with OBD-I/M functionality was legalized.

The primary OBD system could read, record and display error codes after monitoring the exhaust gases of each vehicle.

Now, OBD systems have been expanded and are better equipped at monitoring and controlling vehicle emissions.

Emissions: Monitoring with OBD system checks 

The OBD computers in your car are capable of measuring the exhaust temperature to maximize emission control and safety. Thus, during the OBD smog test, local authorities will run a system check on your vehicle.

All relevant exhaust gases and components are monitored by OBD systems.

As of 2000, the European Union followed the legislation of the US and introduced the EURO III emissions standard or the EOBD.

In short, the goals of OBD systems include:

  • The continual monitoring of exhaust gases
  • The immediate detection and report of error codes that can read an increase in emissions
  • The intention to help avoid escaped gases that have a negative impact on the environment.

OBD plays a vital part in helping each car owner meet the air quality standards required by the EPA and the EU.

Bonus: Best OBD2 Scanners Review and Comparison 2021

OBD Smog Test and Fuel Emissions

After the introduction, in 1996 all cars and some trucks were obligatory to have an OBD II system installation.

Originally, the OBDII system was made to help detect emissions problems.

OBDII systems can give an in-depth read about the quality and chemical components in any vehicles exhaust.

For example, with fuel tank diagnostics the OBD system is capable of detecting a faulty component or function within the fuel tank ventilation system.

Both the fuel tank ventilation system and fuel tank leakage indicators inside OBDII systems can prevent the fuel vapors from escaping and entering the environment.

Instead of releasing the dangerous Hydrocarbons into the air, the activated carbon filter inside the ventilation system can accumulate harmful gas emissions.

Unlike annual emissions tests, the OBD II system in your car detects emissions every time you turn on your vehicle.

OBD Smog Test and the Catalytic Converter

The OBD system pulls together data from a vehicles catalytic converter, misfiring cylinders, oxygen sensor, and coolants to make accurate measures of emissions.

The catalytic converter is one of the most important components of emissions control. Because its main role is to clean exhaust gases.

All in all, the functions of the catalytic converter include:

  • Reducing Nitrogen oxides (NOx) to carbon dioxides (CO2) and nitrogen (N2)
  • Oxidizing Carbon Monoxide(CO) to carbon dioxide (CO2)
  • Oxidizing hydrocarbons to carbon dioxide (CO2) and water (H20)

Because of the fact, the Catalytic converter has a strong influence on emissions control, the OBD system is responsible to monitor this part.

The Effects of OBD Smog Test in action

Car companies have used this newly implemented technology to reduce the carbon emissions in their vehicle makes and models.

Look at this chart that compares Manufacturer Adjusted Fuel Economy and CO2 Emissions:

OBD2 vs Emission Control via OBD Emission Test
MY 2015-2017 Manufacturer Adjusted Fuel Economy and Adjusted CO2 Emissions.
Credit: United States Environmental Protection Agency

Reading OBD Systems

OBDII scanners can provide fast and accurate vehicle data readings to let you know if your car exceeds the federal limit for an emissions test.

This will not only make driving safer but is also an important step in reducing the pollution problem.

When an OBD II scanner runs a system check during the OBD emission test, it will let you know your vehicles readiness by showing flags displayed on the scan tool.

How long does a smog check take?

Normally, a smog check will take you about 30 minutes to finish. The computer will run through all the car systems to detect and give graphic of the data on the computers. Then a complete report will be produced to decide if your vehicle pass the smog test or not.

How to pass Smog Test?

These are some tips to pass OBD Smog Test:

  • Check if your car running properly: Feel your car behind the wheel, if you feel something is not correct, get even a cheap OBD2 scanner to figure it out.
  • Check engine light is off: OBD2 scanners can help you to read and reset trouble code. The malfunction light thus will not be blinking all the time.
  • Change your oil for a smoother engine: this is a typical habit to keep your car healthy. Not only for the engine but also for the emission test. Contaminated oil takes away the clean breath of the engine and makes you fail an emission test.
  • Make sure all tire keeps the car stable and easy to control: the pressure of the tire affects dynomometer when collecting exhaust. Good tires keep your car moving well during the smog test.
  • Ensure the battery is connected: if you by chance change or renew the car battery, reset it with a scanner or wait for a while before you do the test.
  • Temperature: Remember to warm up the all car parts before the test with a 15-minute drive.

In brief, tips to pass emission test? Get yourself an OBD2 scanner!

Bonus: Complete Buying Guide For The Best OBD2 Scanner.


Unfortunately, we don’t all drive a zero-emission vehicle. For it’s unlikely that all the world can magically replace their vehicles with an eco-friendly alternative.

This not-so-simple fix is not only impractical and expensive but would also contribute to the increasing waste problem.

Today, vehicles are highly sophisticated and with just those small OBD2 scan tools, they can help ensure all parts of our vehicles are running smoothly.

From an environmental perspective, the benefit of the OBDII systems is that they monitor and report the status of all vehicle emissions. That’s why it is a prerequisite for all vehicle to pass an OBD emission test; although this policy is not exactly the same in different states.

This is a practical way to ensure every vehicle’s emissions stay within the federal limitations.

In a world filled with lanes full of traffic and heavy-duty diesel engines, it’s important to pave the way for a cleaner future.


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