Video: Smog Testing 101

01:29 min

By Cars.com Editors

October 17, 2017

Kelsey Mays gives an overview of what you need to know about smog/emissions testing in this week’s segment of Driving Smart.

(upbeat music) For millions of Americans, regular emissions tests, sometimes called smog tests, are just another part of vehicle ownership. 31 states and the District of Columbia have at least one county that requires them, according to the EPA.

But, if you just moved into one of those counties or you bought a newer car, these tests might come as a surprise. Not to worry though. With proper maintenance and knowledge of a few warning signs, your car should pass with no problem. So what exactly are these tests? Well, they used to involve putting your car on a dynamometer, or a vehicle treadmill, and hooking up equipment to its tailpipes to measure what's coming out. But nowadays, technicians usually hook up a device to your Onboard Diagnostics 2 port, or OBD2, and check through a number of monitors. These 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 these monitors signaled a problem. That'll often coincide with an illumination of your check engine light, which indicates the OBD2 has detected a malfunction in your car's emissions controls. The reason might be as simple as a loose gas cap, or a serious as a cylinder that's lost compression. Experts tell us that you shouldn't just go to a different testing station, as OBD2 emissions tests are considered reliable and don't have a lot of variants. But if your car is in good working order and you've addressed whatever reason that 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. (upbeat music)