How To Pass a Smog Check

Before you take your car in for a smog check, is there anything you can do to give it a better chance of passing? The obvious answer is to make sure your car is running well in the first place. A well-maintained car, with all its systems operating correctly, will probably pass the smog test.

If you think your car isn't running at 100 percent but you want to avoid the expensive repairs that would be required if you fail, there are simple steps you can take to tilt the odds of passing a smog check in your favor.

We'll get to the details in a minute, but first it's important to understand that smog testing, introduced in the 1970s as part of the Clean Air Act, is an essential step to keeping health-threatening pollutants out of the air. Smog check programs are in effect in 33 states to verify that your car's emissions system is functioning properly. For more information about your local smog check requirements, check with your state's motor vehicle registry.

"I can remember crying during recess in elementary school because the smog levels were so high," says Steve Mazor, chief automotive engineer with the Automobile Club of Southern California. He adds that in the 1960s, Los Angeles had 100 smog alerts each year. In the past 10 years there have been only two in the city. "That is almost entirely because of the improvements in emissions systems in cars," he says.

Still, smog tests can be a bureaucratic hassle for car owners. Understanding the rules, and how to prepare for and take the test, was so confusing for the average motorist that smog check technician Eddy Asmerian created SmogTips.com with information about how to pass.

"Most people leave the smog test until the last minute," Asmerian says. "They think, 'If I don't pass, I'll worry about it then.'" But he says there is a lot they can do ahead of time to help make sure their car will get a clean bill of health.

Here are the top tips from our experts to prepare your car for a smog check:

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.

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.

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.

3. Change the oil, but only if it needs it. Dirty oil in the crankcase could release additional pollutants, which could cause the car to fail the smog test, says Asmerian. While the mechanic is changing the oil, ask him to do a visual inspection of the car's engine to see if any hoses are cracked, broken or disconnected.

4. Do a tune-up two weeks before the smog test. Have any required maintenance performed well before the smog test, Mazor says. Most mechanics disconnect the battery while doing a tune-up and this resets the car's onboard computer. The car then needs two weeks of driving to run all the diagnostic tests needed to pass the smog test.

5. Make sure the tires are properly inflated. Many states require a dynamometer test, which positions the car's tires on rollers that allow the engine to run at high speeds while it is stationary. If the tires are under-inflated, the car's engine works harder to achieve the engine revolutions required by the test.

6. Check coolant and gas levels before the test. Since the test runs the car at high speed while it is stationary, less air flows through the radiator to cool it. So make sure to fill your coolant tank properly. Also, the car will be on a dynamometer, possibly at an angle. If gas is low in the tank, this could expose the fuel pump and put vapor in the fuel line, causing the car to fail the test.

7. Get a pre-inspection. In some states, smog check stations will do a less expensive pre-inspection that shows if a car will pass or fail without officially recording the results with the state's registry of motor vehicles. If an owner knows the car is borderline, this might be a good idea, says Asmerian. Mazor thinks this is unnecessary: "If it fails, have the work done then."

8. Avoid rainy days. There was disagreement on this recommendation. Asmerian says that wet tires can slip on the dynamometer and give a false reading that can cause smog check failure. However, Mazor says the warm-up cycle of the test will usually dry off the tires. Furthermore, extra humidity during rainy days results in lower emissions of some pollutants.

9. Use a fuel additive. Older cars could have clogged fuel injectors, causing them to run lean, says Asmerian. An additive such as Techron could clean the small openings and help the car pass the smog test, he says. Mazor believes this is unnecessary in California, however, since the gas sold in the state is the "best in the world" and loaded with additives that will keep the engine clean.