For millions of Americans, regular emissions tests — sometimes called smog tests — are part of vehicle ownership. Thirty-one states and the District of Columbia have at least one county that requires them, according to the EPA. But if you just bought a newer car or moved into one of those counties, these tests might come as a surprise. Not to worry: With proper maintenance and knowledge of a few warning signs, your car should pass with no problem.
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So, what does an emissions test involve? It used to require putting your car on a dynamometer, or vehicle treadmill, and hooking up equipment to the tailpipes. Nowadays, technicians usually hook up a device to your onboard diagnostics II port to check through a number of monitors. Those are test cycles of various systems in your car, and you'll pass or fail depending how they perform.
Now, if you fail a test, it's because those monitors signaled a problem. That will often coincide with an illumination of your check- Used Engine light, which indicates the OBD-II has detected a malfunction in your car's emissions controls. The reason might be as simple as a loose gas cap or as serious as a cylinder that's lost compression.
Experts tell us you shouldn't just go to a different testing station, as OBD-II emissions tests are considered reliable and don't have a lot of variance. You also shouldn't try to use a diagnostic tool just to off the light without getting the root problem fixed.
But if your car is in good working order and you've addressed whatever reason the light came on — or it's not even on in the first place — you shouldn't lose sleep over an upcoming test. Chances are, your car will pass with flying colors.