How to Pass a Smog Test

Co-authored by Tom Eisenberg

Last Updated: November 16, 2024 References

Smog tests are done to reduce the amount of pollution cars add to the air. Check with your local or state government to see if passing this test is required by law.^{[1]
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Tom Eisenberg
Auto Technician

Expert Interview. 26 July 2019.} To improve your car's odds of passing, take good care of it by repairing problems and changing out the oil. On the day of the test, make sure the car is warmed up before getting to the facility. A failed test means more repairs and more tests, but remember that it ultimately leads to cleaner, healthier air.

Part 1

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Fixing the Car before the Test

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Repair your car if it isn’t running correctly. If you know your car needs repairs, get it fixed before taking a smog test. Find an auto mechanic in your area to get a diagnostic test for your car. Have them fix the issue or repair it yourself so you don’t waste time and money on a failed smog test.
- When the “check engine” light is lit on the dashboard, your car will usually fail the test.
- Problems, such as a faulty oxygen sensor, become more serious and costly when left untreated.
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Drive the car for at least a week after the battery is disconnected. The battery is typically disconnected during repair work. This resets the car’s internal computer, which deletes the computer’s diagnostic reports. Drive the car for 100 to 200 mi (160 to 320 km) over the course of a week or 2 before the test.^{[2]
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- The smog test technician will retrieve the diagnostic reports from the car’s computer during the test.
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Get an oil change if the car needs it. An oil change is only needed if your car is near the next scheduled service. Oil changes are typically recommended every 3,500-5,000 miles for conventional oil or every 7,500-10,000 miles for synthetic oil.^{[3]
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Tom Eisenberg
Auto Technician

Expert Interview. 26 July 2019.} Old oil releases pollutants that can cause your car to fail the test.^{[4]
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- If you have a technician do it, ask them to look for cracks in the car’s hoses.
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Check the engine’s coolant level. This is done by opening the car’s hood and unscrewing the radiator cap. The fluid level should be near the top of the tube. During the test, your car will stay in place and have less air flowing to cool it.^{[5]
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- This fluid may be replaced during any repair work or oil change.
- Coolant helps prevent your engine from overheating.^{[6]
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  Tom Eisenberg
  Auto Technician
  
  Expert Interview. 26 July 2019.}
Pre-inspections are done at the same facilities as the official tests. Ask the technicians to have a pre-inspection. They’ll give your car the official smog test, but the results won’t be reported to the government. This is cheaper than the official test and should be done when you’re unsure the car will pass it.^{[7]
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Part 2

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Taking the Smog Test

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Inflate the car’s tires before the test. Many smog tests include a dynamometer test, which rotates your tires at high speeds. Most gas stations have air pumps you can use to fill the tires.^{[8]
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- Properly inflating the tires means less stress on the engine during the test since your car will be more stable.
Some dynamometer smog tests hold the car at an angle. This exposes the fuel pump, which can mean a test failure due to more vapor in the fuel line. Keep the tank as full as possible before going to the testing facility.^{[9]
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Wet tires can cause the car to slip off the dynamometer, leading to a false reading and failure. The testing machines will dry your tires, so you can still complete the test on a rainy day. However, it’s better to avoid the risk and wait for better weather.^{[10]
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Use fuel additives to clean the fuel lines. Fuel additives can be purchased at auto parts stores. Follow the directions on the bottle to add the additives to the car’s gas tank. This is usually done during refueling.^{[11]
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- Most gas is already treated with some additives. However, extra additives can help your car pass the test.
If the testing facility is around the corner from your house, avoid going there immediately. The drive will warm up your engine, ensuring that it’s operating as efficiently as possible.^{[12]
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Take the test at a registered facility. Go to your area or state government’s website. Certified stations are operated all over the area, so you most likely won’t have a problem finding one close to you. Some of the stations may even be able to repair your car after the test.^{[13]
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- For tests in California, for example, go to https://www.bar.ca.gov/smog-checklist
At the end of the test, the technician will give you a printout. Hand the printout to a repair shop so the technicians there can diagnose your car and make any necessary repairs.
After a failed test, you’ll usually be given a few weeks to fix the car without penalty. Adjust the car as needed, then return to the testing facility. Continue doing this until your car officially passes the test.^{[14]
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Question

What is checked during a smog check?

Tom Eisenberg
Auto Technician
Tom Eisenberg is the Owner and General Manager of West Coast Tires & Service in Los Angeles, California, a family-owned AAA-approved and certified auto shop. Tom has over 10 years of experience in the auto industry. Modern Tire Dealer Magazine voted his shop one of the Best 10 Operations in the Country.

Tom Eisenberg

Auto Technician

Expert Answer

During the test, someone will hook up a meter to your car's exhaust and engine and make sure your vehicle's catalytic converter is doing its job.

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What happens if I fail a smog test?

Tom Eisenberg
Auto Technician
Tom Eisenberg is the Owner and General Manager of West Coast Tires & Service in Los Angeles, California, a family-owned AAA-approved and certified auto shop. Tom has over 10 years of experience in the auto industry. Modern Tire Dealer Magazine voted his shop one of the Best 10 Operations in the Country.

Tom Eisenberg

Auto Technician

Expert Answer

If you fail, you'll have to get your vehicle repaired until your emissions are at a reasonable level. You can then get your vehicle tested again.

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Question

How often do I need to get a smog test?

Tom Eisenberg
Auto Technician
Tom Eisenberg is the Owner and General Manager of West Coast Tires & Service in Los Angeles, California, a family-owned AAA-approved and certified auto shop. Tom has over 10 years of experience in the auto industry. Modern Tire Dealer Magazine voted his shop one of the Best 10 Operations in the Country.

Tom Eisenberg

Auto Technician

Expert Answer

The frequency depends on what state you live in. Check with your state's DMV for more information.

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