1. Clear that "Check Engine" light.
If your car displays a "Check Engine" light, that's an automatic smog check failure. You'll need to get a diagnosis and fix before you test.

The most common reason for a Check Engine light is a faulty oxygen sensor, says Kristin Brocoff, director of corporate communications for CarMD.com, which sells an automotive diagnostic tool and provides repair information. Sometimes, even before an oxygen sensor fails, it becomes "lazy," not properly regulating the gas/air mixture, and that will cause a smog check failure, Mazor says.

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An oxygen sensor in an older car is a $168 part, according to CarMD data. Replacing it is a good idea. Ignoring it can lead to a more costly catalytic converter repair, which can cost more than $1,000.

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2. Drive the car at highway speeds for the two weeks prior to the smog test. This gets the catalytic converter hot enough to burn out any oil and gas residues. The catalytic converter, mandated by federal law in 1974 for all U.S. cars and trucks, converts harmful pollutants into less harmful emissions before they leave the exhaust system. The worst thing for the proper operation of emissions systems is a series of short trips: The catalytic converter never gets hot enough to do its job, Mazor says.

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3. Stay Up to Date with Oil Changes

Clean oil in your vehicle is important for any inspection, but specifically for smog testing because dirty oil in your car’s engine will emit extra pollutants that are bad for the environment and for your smog test. For even more security, it’s a good idea to ask your mechanic to visually inspect your engine to make sure all hoses are holding up well and intact.

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4. Stay on Top of  Maintenance

To pass the smog inspection or basically any car inspection is to follow a strict car maintenance routine just to make sure nothing goes wrong in the first place. If you haven’t done so, it’s a great idea to get into a shop for a tune-up at least 2 weeks prior to your smog test. When your mechanic checks your car, he will find any issues that may contribute to a smog test failure, and he will probably also unplug your car’s battery. Most cars need at least 2 weeks of driving after a battery disconnection to run all of the diagnostics necessary to pass a smog test.

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5. Make sure Tires are Fully Inflated

While your car is being tested, your tires will likely be placed on rollers to make the vehicle run at high speeds to check emissions. If your tires aren’t fully inflated, your engine is forced to work harder to run at high speeds, running the risk of emitting dangerous fumes.

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6.More Tips are on their way