A smog check is an emissions test that measures how much pollution your vehicle produces

A smog check is a test that checks whether your car, truck, or motorcycle meets your state's air quality standards. The test measures the exhaust coming from your vehicle's tailpipe to see if it contains too much nitrogen oxide, particulate matter, or other pollutants. If your vehicle passes, you get a certificate. If it fails, you cannot renew your registration until you fix the problem and pass again.

Not every state requires a smog check. California, New York, Massachusetts, Vermont, Maine, and a few others run smog check programs. Some states test only vehicles in certain counties or regions where air quality is poor. Your state's Department of Motor Vehicles or environmental agency can tell you whether your vehicle needs one.

The test itself takes about 10 to 15 minutes and costs between $30 and $80, depending on your state and the testing station. You do not need an appointment at most places — you can walk in during business hours. The test is performed by a technician at a licensed smog check station, which is usually an auto repair shop or a dedicated testing facility.

Key Takeaways

  • A smog check measures your vehicle's exhaust to confirm it meets state air quality standards, and you need a passing certificate to renew your registration.
  • Only certain states require smog checks, and some states test only vehicles in specific regions or counties with air quality concerns.
  • The test takes 10 to 15 minutes, costs $30 to $80, and is performed at licensed testing stations that are usually auto repair shops.
  • If your vehicle fails, you must repair the problem and retest before you can renew your registration or sell the vehicle.

When you need a smog check

Most states that require smog checks ask you to get one every two years when you renew your vehicle registration. Some states test every year. A few states test only when you first register a vehicle in that state or when you transfer ownership to someone else.

Your registration renewal notice will tell you whether a smog check is required. If your state requires one, you cannot complete your registration renewal online or by mail until you have a passing smog check certificate. You must take the certificate to the DMV or upload it to your state's online registration system.

Older vehicles are sometimes exempt from smog checks. In California, for example, vehicles manufactured before 1975 do not need to pass. Some states exempt vehicles that are 25 or 30 years old. Check your state's rules — the DMV website will list which vehicles are exempt.

What happens during the test

When you arrive at a smog check station, bring your vehicle registration and a photo ID. The technician will ask for your vehicle's make, model, year, and mileage. They will then connect a probe to your vehicle's tailpipe and run the test, which measures the levels of carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter in your exhaust.

The test is non-invasive — the technician does not take apart any part of your vehicle. For most vehicles, the probe straightforward sits in the tailpipe while the engine runs at idle and at a higher speed. The machine records the readings and compares them to your state's standards for your vehicle's year and type.

The results are when ready. If your vehicle passes, the technician prints a certificate right away. If it fails, the certificate will list which pollutants exceeded the limit. You then have a set number of days (usually 30 to 60) to repair your vehicle and retest. Some states allow you to retest at no charge if you fail the first time.

What causes a smog check failure

The most common reason for failure is a faulty oxygen sensor, a problem with the catalytic converter, or an issue with the fuel injection system. These parts work together to control how fuel burns in your engine. When one fails, unburned fuel or excess nitrogen oxides escape into the exhaust.

A check engine light on your dashboard is often a sign that your vehicle will fail a smog check. The light means your vehicle's onboard diagnostic system has detected a problem. You can take your vehicle to a repair shop and ask them to read the diagnostic code before you pay for a smog check — this tells you what needs to be fixed.

Vehicles that are well-maintained and have no warning lights usually pass without trouble. Regular oil changes, air filter replacements, and tune-ups reduce the chance of failure. If your vehicle is older or has high mileage, have a mechanic inspect it before you pay for a smog check.

What to do if your vehicle fails

If your vehicle fails, take the failed certificate to a repair shop. The certificate lists which pollutants were too high, which gives the mechanic a starting point. Common repairs include replacing the oxygen sensor, cleaning or replacing the catalytic converter, or fixing a fuel leak.

Repair costs vary widely depending on what is wrong. A new oxygen sensor might cost $100 to $300. A catalytic converter replacement can cost $500 to $2,500 or more. Before you authorize repairs, ask the shop for an estimate and ask whether they offer a warranty on the work.

After repairs are complete, you must retest at a smog check station. Some states allow you to retest at the same station at no charge within a certain time frame. Bring your repair receipt and your failed certificate to the retest appointment. Once you pass, you can complete your registration renewal.

How smog checks affect vehicle sales

If you are selling a vehicle in a state that requires smog checks, you must provide a passing smog check certificate to the buyer. The certificate is usually valid for 30 days, so you should get it close to the sale date. Some states allow the buyer to get the smog check instead, but the seller is responsible if the vehicle fails.

A vehicle with a failed smog check cannot be sold until it passes. This is why it is important to address any check engine light or emissions problem before you list the vehicle for sale. Buyers will often ask for proof that the vehicle passed, and a failed certificate will make the sale much harder.

Frequently Asked Questions

Can I drive my vehicle if it fails a smog check?

Yes, you can still drive it. A failed smog check does not prevent you from driving — it only prevents you from renewing your registration. However, you should repair the problem as soon as you can, both to pass the test and to avoid further damage to your engine.

What if I just moved to a state that requires smog checks?

When you register your vehicle in a new state, that state will tell you whether a smog check is required. Some states require one when you first register an out-of-state vehicle. Check your state's DMV website or call them to find out the requirement for your situation.

Do hybrid or electric vehicles need a smog check?

Hybrid vehicles may need a smog check depending on your state's rules — check your state's DMV website. Most states exempt fully electric vehicles from smog checks because they produce no tailpipe emissions. Plug-in hybrids may be treated differently depending on the state.

How long does a smog check certificate last?

A passing smog check certificate is usually valid for 30 days. You must complete your registration renewal within that time frame. If the certificate expires before you renew, you will need to get a new smog check.

What if my vehicle is too old to test?

Some states exempt vehicles over a certain age — typically 25 to 30 years old — from smog checks. Check your state's DMV website to see if your vehicle qualifies for an exemption. If it does, you will not need a smog check to renew your registration.