How To Make Sure Your Car Passes a Smog Check

There’s no magic involved in passing a vehicle emissions test or simply smog text or smog check; it’s pure science. … Continued

There’s no magic involved in passing a vehicle emissions test or simply smog text or smog check; it’s pure science. Understanding how emissions are created by your car’s engine and how to maintain these emissions to within what is set by the government as the normal range can help you pass your vehicle’s smog check with flying colors.

pollution test

First, a Look at the Smog Check

Vehicle emissions testing is part of the 1970 Clean Air Act that sought to regulate air emissions from both mobile and stationary sources. The law also empowers the Environmental Protection Agency to create the National Ambient Air Quality Standards so that the general public can be protected from hazardous air pollutants. It was amended in 1977 and then in 1990.

The state of California was given the leeway to create its own emissions standards because of the severe air pollution in the LA metropolitan area due to motor vehicles. It initiated the California Smog Check program in 1984, although this was preceded by an inspection and emissions inspection program in New York in 1982.

Other states have the option to follow either the federal EPA guidelines for vehicle emissions standards or the California model. Currently, there are 13 states that patterned their emissions standards with that of the state of California. These include Washington, New Mexico, Oregon, New Jersey, New York, Massachusetts, Connecticut, Pennsylvania, Vermont, Maine, Delaware, Rhode Island, and Maryland. The District of Columbia also observes the California standards. Twelve states do not require emissions testing. These include Alaska, Montana, North Dakota, South Dakota, Iowa, Kentucky, Mississippi, Arkansas, Michigan, Minnesota, South Carolina, and Florida.

Majority of the states that require emissions testing conduct the test every one or two years, although not all vehicles will be subjected to such a test. For example, in California, cars that are no more than 6 years old are exempted from such a test. In New York, what are exempted are vehicles that are no older than 2 years, are electric-powered, are registered with historic plates, or are older than 26 model years.

How the Test is Conducted

During a smog check, your vehicle will be inspected and evaluated by a licensed technician. There are 3 parts of the test: emissions, visual, and functional inspections. The aim is to make sure that your car’s emissions system is properly doing its job. Emissions tests measure a number of gases such as carbon monoxide, hydrocarbons, carbon dioxide, and oxygen. In heavily-polluted areas, oxides of nitrogen are also measured.

Depending on the state where you live, the smog check can be performed using any or a combination of different methods. The Two-Speed Idle test is usually performed on cars that were registered before 1995. The engine is left idling at high speed then at low speed. The exhaust emissions are then carefully evaluated.

The Acceleration Simulation Mode is also used on older vehicles, especially those without an onboard diagnostic computer. The car is evaluated using a dynamometer. The exhaust emissions are evaluated under simulated driving conditions.

Vehicles with an On-Board Diagnostics (OBD) module are tested using the car’s own data. A device is plugged into the car’s OBD port and the data is downloaded for analysis.

car diagnosing tool

Passing the Smog Check

Given that the smog check is an evaluation of your car’s exhaust system which is directly related to the functioning and performance of your engine, passing with flying colors should be relatively easy. Here are some tips to get you that passing mark.

  • Fix Your Car Before the Smog Check

Even before you take your car for an emissions test, make sure that it is in good working condition. After all, this is what the test will be evaluating. If you see your Check Engine light coming on from your dashboard, don’t ignore it. Have a professional identify what is causing the Check Engine light to turn on. Alternatively, you can try to turn this warning light off, but is generally not recommended. One of the most common culprits for a Check Engine light is a problem in the oxygen sensor.

  • Drive at Highway Speeds at least 2 Weeks Prior to the Test

If you are already driving at highway speeds every single day, then you can skip this part. However, if your typical day usually involves relatively short trips, make sure to take your vehicle for a very long drive at least 2 weeks prior to the test. This will help clear out the catalytic converter. Short trips have this nasty effect of not allowing the catalytic converter to become hot enough to burn out gas and oil residues in the exhaust. Taking it for a long drive should help clear the problem.

If you had your car repaired a few weeks prior to the test, it is also recommended that you drive it for at least 200 miles before having it checked. When your car was repaired, the battery was disconnected. This resets the internal computer of your car. If your smog check involves plugging into your car’s OBD port, then there must be data to download. That’s why you have to drive it for at least 200 miles about 2 weeks prior to the test.

  • Consider Having a Pre-Inspection Test

This is not really necessary, but it should give you a heads-up on the current status of your vehicle. Pre-inspection tests are also administered by the same facilities that conduct the official tests. The only difference is that the report is not submitted to the government. This way, you can check if there are still some things that need to be fixed. Take note of the main issues identified in the test. If you’re not sure what they mean, you can ask the testing facility for advice on what you can do to help improve the test results. Consider this option only if you are not sure about passing the official vehicle emissions test.

  • Inflate Your Car’s Tires to the Higher Limit of its Pressure Range

If your car will be inspected using a dynamometer you may want to inflate your car’s tires to the maximum limit of the allowable pressure. For example, if the tire manufacturer says it can be inflated from 32 to 40 PSI it is best to inflate your tires at 40 PSI. This allows your car’s engine to turn the wheels more efficiently. In other words, you will be reducing the work of your engine, resulting in cleaner exhaust.

  • Get a Full Tank

Some testing facilities put the car at an incline on the dynamometer. This can expose the fuel pump, allowing more vapors to form in the fuel line. As much as possible, fill your fuel tank to its maximum capacity on the day of the smog check itself.

  • Use the Right Fuel Additives

Some car owners don’t believe in the value of fuel additives. However, these simple solutions could just be the answer to passing the smog check. Fuel additives clear carbon deposits that may be present in the engine intake and exhaust, allowing for the more efficient flow of both fuel and air. This also leads to lower emission levels while improving overall engine performance.

Using fuel additives is pretty straightforward. Drive your car around until the fuel gauge on your dashboard is almost at the empty level. Drive to the next gas station and pour the fuel additives into your fuel tank. Fill it up with the right fuel. Drive as normally as possible. When your fuel tank is nearly empty, get a full tank of the right fuel. Now take your car for it to be tested.

  • Warm Up Your Vehicle Before the Smog Check

Before you take your car to the smog testing facility, make sure to drive it for about 20 to 30 minutes first. This is to make sure that the engine is performing as efficiently as possible. Keep in mind that as the engine gets to the optimal working temperature, there is less chance of hydrocarbons appearing out through the tailpipe.

Ideally, you would want to make sure that your car will get smog tested as soon as you drive it into the testing facility. You do not want to be on the end of a long queue where you will need to turn off your engine and allow it to cool down. It is best to make an appointment with the testing facility so you don’t have to run the risk of turning off and cooling down your engine simply because there’s a long line of vehicles being tested.

It is also advisable to bring your car to be tested on a cool, but not rainy, day. Cool air allows your engine to run more efficiently. This can help provide for more positive emissions test results.

Passing your vehicle’s smog check does not involve magic or luck. There are no tricks or shortcuts to passing such tests. The only way you can pass successfully every time is by keeping tabs with your car’s maintenance schedule and driving sensibly.


  1. What Is a Smog Check? – Angies list
  2. How to Get a Smog Check (CA) – wikiHow
  3. How to Pass a Smog Test – wikiHow
  4. How to Pass Emissions – wikiHow