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How to "Cheat" an Emissions Test

Updated on November 15, 2017
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Kate is a research enthusiast with a Bachelor of Science degree from Sonoma State University. Kate has a passion for the automotive field.

When we say "cheat an emissions test" (i.e. smog check), we aren't referring to passing the test illegally. On the contrary, we are referring to techniques that can be employed to assist you in passing a smog test 100% legally! These are the most common techniques to getting a vehicle that may otherwise fail an emission (smog) test, to pass with flying colors! Nearly everyone is nervous prior to his or her smog test, so let us give you a little peace of mind: If you perform all of these techniques, I can tell you with 95% certainly that you will pass your smog test (yes, even you California residents with the strictest of requirements)!

Best Practices for Passing an Emissions (Smog) Test:

1. Test a very warm car on a cool day.
4. Perform an oil change.
2. Use a specialized fuel additive.
5. Turn your "Check Engine" light off (if needed).
3. Inflate tires to the proper (high) level.
6. Find a shop that offers a free retest.

1. Warm your car's engine up as much as possible.

Your car's engine burns fuel, to make sure there is complete combustion (i.e. the engine is running as clean as possible) your engine needs to warm-up. The warmer your vehicle's engine is, the better. If you want to do even better, test your car on a relatively cool day while still making sure your engine is as warm as possible.

Drive around for at least twenty minutes before getting your car tested. If it takes you twenty minutes to reach the station, you are good to go!

2. Use a fuel additive.

A fuel additive is usually poured into your car’s gas tank while you are refueling. The purpose of specialized fuel additive is to assist with cleaning any carbon deposits that are in your engine’s exhaust and intake paths. This allows for air and fuel to flow easily through these passages. This lowers your emission levels and improves combustion and your engines overall performance.

I have personally used CRC "Guaranteed to Pass Emissions Test Formula" at least ten times on my personal vehicles and true to the name, I have never failed a smog test on any of my cars. Just pour the entire bottle into your gas tank when refueling, run your vehicle until your gas tank is near empty, then refill your gas tank again and take your test! The company claims that you are guaranteed to pass your smog test "or double your money back." I have never put that claim to the test because so far all my vehicles have passed with this stuff. Even if your vehicle has a bad catalytic converter, you may still pass with this specialized additive. Just be sure to closely follow the instructions on the label and you should be fine.

3. Be sure your tires are inflated to the proper (high) level.

During your smog inspection, the technician may have to drive your car on what is called a ‘dynamometer’ as part of the testing process. Fill your tires to the correct pressure prior to the smog test. If your tire manufacturer states that your tires can be safely inflated to any pressure within a given range (for example: anywhere between 35 and 45 PSI), then inflate them to the top of that safe range. The slightly higher tire pressure will allow your engine to turn the wheels with less effort. Meaning your vehicle will run slightly cleaner at 25 MPH with tires inflated to 45 PSI versus tires that are inflated to 35 PSI.

An auto mechanic prepares a vehicle for an emissions test.
An auto mechanic prepares a vehicle for an emissions test.

4. Change your oil prior to your smog check.

The positive crank ventilation (PCV) system of your car has been specially designed to let your engine breathe the fumes that are located in the oil compartments. These fumes are burned during the combustion process. If your engine’s oil is contaminated because of inadequate oil changes, it could cause higher emissions and a failed smog test.

For these reasons, always change the oil in your car to ensure the best engine performance prior to your test.

An illuminated "Check Engine" light will cause you to fail your emissions test.
An illuminated "Check Engine" light will cause you to fail your emissions test.

5. If your check engine light is illuminated, you need to find a way to turn it off.

A check engine light/malfunction indicator lamp that is illuminated will automatically cause you to fail the smog test. If you take your car to a certified repair technician, he should be able to diagnose the reason why your check engine light/MIL is illuminated and give you an estimate to have the problem fixed. This is usually done for free.

There are also many simple techniques to turn off a check engine light that can be attempted prior to your smog test. These techniques are too numerous to discuss here, but a quick search for "how to get rid of the check engine light" should give you some things to try. Some techniques may only turn off the check engine light for a short time, so make sure you get your smog test done soon after the light goes off!


When you go to have your vehicle smog checked, have as many cards stacked in your favor as possible. If you utilize all of these tips, I can just about guarantee that you will have no problem passing your emissions test.

If you are at all worried, put in a little extra effort and find a smog station that offers a free retest if you fail your smog check the first time. Smog test stations are getting a lot more competitive with each other in recent years and it is not uncommon to find this deal if you call around. It could save you quite a bit of money on a retest.

Please comment below with your own tips/tricks to passing a smog test to help other readers!

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