What Is Car Smog and How Do I Get It Checked?

You've heard of smog, but do you really know what it is? We'll clear up the misconceptions and explain what you should know about testing.

Car smog is the excess exhaust and other harmful health hazards and environmental pollutants that escape from your vehicle into the atmosphere. According to the United States Environmental Protection Agency (EPA), the U.S. transportation sector annually accounts for 28 percent of all airborne pollutants released into the atmosphere.

The introduction of On-Board Diagnostics, Second Generation (OBD II) considerably reduced the amount of smog (a derivative of smoke and fog) produced and released by internal combustion engines. However, with 273 million vehicles (from cars to big rigs) on the road, and Americans driving more than three trillion (yes, trillion) miles a year (2018), on average, each vehicle on the road produces approximately 4.6 metric tons of smog contaminates each year.

Follow along to learn how vehicle smog is produced and what you need to know to pass a smog test.

What Is Car Smog?

Car smog is a product of burned and unburned vehicle pollutants. The four basic types of emissions/pollutants are: hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOX) and solid particle (carbon, ash, fuel additives). Mixing with oxygen, nitrogen and other compounds in sunlight, airborne pollutants produce visible photo-chemical smog and ground level ozone.

These are the sources and percentage of auto smog impurities.

  • 20 percent from crankcase vapors
  • 20 percent from fuel evaporation
  • 60 percent from an engine’s tailpipe

How Does Car Smog Occur?


Excess hydrocarbons are the result of unburned fuel, dirty motor oil saturated with combustion by-products and poor crankcase ventilation. Common causes of high hydrocarbons are a raw fuel leak, normal fuel evaporation, bad fuel cap or vapors that escape while filling the fuel tank. Common mechanical or electrical failures include a clogged PCV valve, leaking fuel injector, defective fuel pressure regulator, a failed engine sensor or ignition component that results in incomplete combustion allowing raw, unburned fuel to make its way into the exhaust system and out the tailpipe. Most likely you’ll experience poor gas mileage and performance, as well as possibly damaging the O2 sensor and catalytic converter.

Carbon monoxide

Carbon monoxide is a deadly, colorless, odorless toxin that is also a smog by-product of incomplete combustion or a rich air/fuel mixture. Frequent causes of excess carbon monoxide are a dirty air filter, faulty oxygen sensor, manifold pressure, throttle position or coolant sensors.

STAY SAFE! Even if your engine is running great, never drive a car with a defective or leaking exhaust system, or if the car body, trunk or floor pan has holes in them, and never, ever leave your car idling in a closed garage. Car exhaust seeping into your car through rust holes, or holes punched in the body to run wires for accessories, contains poisonous chemicals, including carbon monoxide, that can be deadly if allowed to accumulate in the passenger compartment.

Oxides of Nitrogen

Oxides of nitrogen are produced by extremely high temperatures during combustion. Common causes of high NOX are a failed or failing catalytic converter or exhaust gas recirculation (EGR) valve. High compression engines, engines that run hot or a lean running engine also produce excess NOX. However, these factors also lead to increased fuel mileage while reducing hydrocarbons and carbon monoxide smog.

Solid particles

Solid particulates are visible as black smoke exiting the tailpipe. Some are heavy enough to settle to the ground, while most can make their way into the atmosphere. Common causes of smog/black smoke are fuel or ignition system problems, a failing or failed engine sensor or a severely worn engine burning oil in the combustion chamber. Smog particulates can build-up inside your engine causing all types of drivability issues that increase tailpipe smog levels, as well as causing premature engine wear and damage.

How To Get Your Car Checked for Smog

Check your state’s Department of Motor Vehicles (DMV) website to determine if your vehicle needs a smog test. Depending on your vehicle’s make, model, year and miles driven in the past 12 months, you may or may not need a smog test.

Many states require a smog check before or immediately after selling a vehicle. If you’re due for a smog check, use the DMV site to locate an approved testing site. If your repair shop is not an authorized smog testing site, you may want to check with the local Better Business Bureau or review trustworthy web sites for reviews from others on how they were treated at a particular testing facility. Make an appointment for the test and remember to bring your driver’s license and vehicle registration.

How To Pass a Car Smog Inspection

Perform basic maintenance approximately 14 days before your scheduled smog test, including:

  • Service the battery, clean the terminals and add battery fluid if needed. A weak battery or charging system will affect ignition and emission sensor performance.
  • Change the oil and filter to remove excess crankcase combustion by-products.
  • Flush the coolant and replace the thermostat to ensure your car is running at peak operating temperature.
  • If your car is running rough, perform a tune-up, including checking spark plugs and wires, replacing the air filter and PCV valve, fuel filter and cleaning the throttle body. Also consider adding fuel injector cleaner to the fuel tank. After adding the cleaner, be sure to drive your vehicle until the fuel tank is down to one-quarter full and refill it before the test.
  • Set tire pressure, especially if the smog check requires a dynamometer test.
  • Replace the gas cap if your car is more than five years old or has more than 50,000 miles.
  • A vehicle will not pass a smog test if the Check Engine Light (CEL) is on. In addition to turning on the CEL, the computer stores a “trouble code” in its memory. Many big box auto parts stores will read the codes for free to help identify the source of the problem.
  • Make sure the exhaust system isn’t leaking and the catalytic converter is in place.

My Car Failed the Smog Test, Now What?

You should receive a comprehensive report explaining why your vehicle failed the smog test. The report may also suggest what repairs are necessary to make your car or truck compliant. Ask your mechanic for an estimate, including diagnostic fees, for the repairs your vehicle needs to pass the smog test. You may consider getting a second opinion. Repairs to pass a smog test can be complex (expensive) or something simple.

If the repairs are prohibitively expensive, ask if your state offers a one- or two-year emission waver and if there is a minimum or maximum amount you’ll need to spend before they’ll issue a waiver.

The Last Word

Getting and keeping smog levels within manufacturers specifications and government regulations is a delicate balancing act of getting several different mechanical, electronic and electro-mechanical parts and components all working precisely together. Most smog-causing problems can be avoided by following your vehicle manufacturer’s maintenance schedule. A well-maintained vehicle has improved fuel economy, decreased smog output — good for the wallet and environment — and helps ensure your driving pleasure.

Bob Lacivita
Bob Lacivita is an award-winning ASE and General Motors auto technician, vocational educator, Career and Technical Center administrator and freelance writer who has written about DIY car repairs, vehicle maintenance and other self-help topics for more than 20 years.
At the age of 12, Bob took his first engine apart, a 2-cycle Briggs and Stratton from a lawn mower he found in the trash. At 14, he rebuilt a seized 256cu.in. Chevrolet engine in a 1956 Belair that he drove for three years. He spent most weekends, as well as the money he earned working a gas station, at Atco Dragway in Atco New Jersey.
Although trained as an architectural drafter, he never worked a day in that field. Still, the skills he learned helped as he renovated and rehabbed his homes. His true love was cars and so he made that his life’s profession. Bob worked for one of the largest Oldsmobile retailers in the country and earned Pontiac and Oldsmobile Master Technician Elite status as one of the top 20 GM technicians in the country.
Bob was also a Pennsylvania Department of Education (PDE) certified career and technical educator for 25 years, teaching automotive technology for 11 of them. He's been a Certified Vehicle Safety Insructor and an Emissions Inspector, too. Bob earned his master’s degree in educational leadership, as well as his PDE K-12 Principal Certification and his Career and Technical Education Directors and Curriculum Supervisors certificates, to become a school administrator. When it comes to education, Bob has two sayings: The kids are the best part of teaching, and teaching was the hardest job he ever had. It was the best job he ever had, too.
Since retiring, Bob has continued to maintain his ASE Master Technician; MACS Section 609 Refrigerant Recycling Certification; PA safety and emissions inspector certifications, credentials, and licenses; and participated in more than 100 hours of update technical training through MotorAge, Snap-On, Dorman Products and Automotive Technician Training Services, Mitchel1 and others.
Bob currently writes regularly for Family Handyman and works as a consultant with one of the largest automotive retailers on the East Coast, setting up an automotive technology training and apprenticeship program in partnership with a local catholic high school.
Bob and his wife lived through 40 years' worth of DIY home remodeling while parenting two (now grown) boys, and now relax by watching their three fabulous granddaughters.