How a Smog Check Works

A smog check is an emissions test that measures whether your vehicle's exhaust meets California air quality standards. The test itself takes about 10 to 15 minutes and costs between $30 and $80, depending on your vehicle type and the testing station. You drive into a bay, a technician connects a probe to your tailpipe, and a machine reads the emissions your engine produces.

The test checks for nitrogen oxides, carbon monoxide, and particulate matter — the pollutants that contribute to smog. Your vehicle either passes or fails. If it passes, you receive a certificate that you take to the Department of Motor Vehicles (DMV) when you renew your registration. If it fails, you have options: repair the vehicle and retest, or in some cases, request a waiver if repairs are too expensive.

Key Takeaways

  • A smog check measures your vehicle's exhaust emissions and takes 10 to 15 minutes at a licensed testing station.
  • You need a passing smog check certificate before the DMV will renew your registration in California.
  • If your vehicle fails, you can repair it and retest, or request a repair cost waiver if the bill exceeds a certain threshold.
  • Smog checks are required every two years for most vehicles, though some newer cars and electric vehicles are exempt.
  • You can find a licensed testing station through the Bureau of Automotive Repair website or by searching for "smog check" in your area.

When You Need a Smog Check

California requires a smog check when you renew your vehicle registration, with some exceptions. Most vehicles need one every two years. Vehicles from 1975 and newer must be tested, but electric vehicles and plug-in hybrids are exempt. Vehicles that are six model years old or newer may also be exempt, depending on the year — the rules change annually, so check with your local DMV office or the Bureau of Automotive Repair website to confirm your vehicle's status.

If you just bought a used vehicle in California, you need a smog check within 90 days to register it. If you move to California from another state, you need one before you can register your vehicle here. The timing matters: you cannot get a smog check more than 90 days before your registration expires, so plan accordingly when your renewal is due.

Finding and Choosing a Testing Station

Any station licensed by the Bureau of Automotive Repair can perform a smog check. You can search for licensed stations on the Bureau's website by entering your zip code — the search tool shows you which stations are nearby, their hours, and whether they handle your vehicle type. Some stations specialize in certain vehicles (motorcycles, heavy trucks, or diesel engines), so confirm before you go.

Prices vary between stations, so calling ahead to ask the cost is worth your time. Some stations charge extra for certain vehicle types or if your vehicle fails the first time. Ask whether the station offers repairs on-site; some do, which can save you a trip if your vehicle fails and you want to fix it when ready. Bring your vehicle registration and a valid ID when you go.

What Happens If Your Vehicle Fails

A failed smog check means your vehicle's emissions exceed California standards. The testing station gives you a report showing which emissions were too high. You then have two paths: repair the vehicle and retest, or request a repair cost waiver.

To retest, take your vehicle to a repair shop, have the problem fixed, and return to any licensed testing station for another test. The retest costs the same as the initial test. You have until your registration expires to pass, so if you fail early in the renewal cycle, you have time to diagnose and fix the problem.

If repairs are expensive, you may be able to request a waiver. California allows a waiver if the cost to repair your vehicle exceeds a certain percentage of its value — the threshold changes yearly but is typically around 30 to 50 percent. The testing station can tell you whether your vehicle qualifies and how to request one. A waiver lets you renew your registration even though your vehicle failed, but you cannot use it if you have already spent money on repairs.

Preparing Your Vehicle for a Smog Check

You do not need to do much before a smog check, but a few straightforward steps improve your chances of passing. Make sure your engine is in good working order: change the oil and oil filter if they are overdue, replace a worn air filter, and fix any obvious problems like a loose gas cap or a check engine light that is on. A vehicle with a check engine light will automatically fail, so address that first.

Drive your vehicle normally for a few days before the test — the engine needs to be warmed up and the computer system needs to be in its normal operating state. Do not try to "clean out" your engine by driving hard or using fuel additives; that rarely helps and can sometimes cause problems. Bring your registration and ID, and arrive with a full tank of gas if possible, since some tests require the vehicle to be driven during the inspection.

Smog Check Exemptions and Special Cases

Electric vehicles and plug-in hybrids do not need smog checks because they produce zero tailpipe emissions. Vehicles that are six model years old or newer may be exempt, though the exact year changes annually — check the Bureau of Automotive Repair website or call your local DMV to confirm whether your vehicle qualifies.

Vehicles registered as historic or collector vehicles may have different rules. Motorcycles and heavy trucks have their own testing procedures and may be tested at different stations. If you own one of these vehicle types, ask the testing station whether your vehicle needs a standard smog check or a different test.

After You Pass: Getting Your Certificate to the DMV

Once your vehicle passes, the testing station gives you a smog check certificate. This certificate is valid for 90 days, so you have that window to submit it to the DMV along with your registration renewal. You can renew your registration online, by mail, or in person at a DMV office — the smog check certificate must be included with your process.

If you renew online, you may be able to upload the certificate or provide the station's reference number. If you renew by mail or in person, bring the physical certificate. Keep a copy for your records. If your certificate expires before you renew your registration, you will need to get a new smog check.

Frequently Asked Questions

Can I get a smog check before my registration expires?

Yes, but not more than 90 days before your registration expires. The certificate is valid for 90 days, so if you get one too early, it will expire before you can use it at the DMV. Check your registration renewal date and plan your smog check for the 90 days before that date.

What if I fail the smog check twice?

You can retest as many times as you need to pass. Each retest costs the same as the initial test. If repairs are not working, ask the testing station or a repair shop to diagnose the problem more thoroughly — sometimes the issue is not what the first test suggested.

Do I need a smog check if I am buying a used car from a private seller?

Yes, if you are registering it in California. You have 90 days from the purchase date to get a smog check and register the vehicle. The seller does not need to provide one, but you cannot legally drive the vehicle without a valid registration, which requires a passing smog check.

Can I get a smog check waiver if I just bought the vehicle?

A waiver is based on repair cost, not on when you bought the vehicle. If the cost to repair your vehicle exceeds the state threshold (typically 30 to 50 percent of its value), you may be able to request one. The testing station can tell you whether your vehicle qualifies.

What if the testing station made a mistake on my smog check?

If you believe the test was wrong, you can request a retest at a different licensed station. The second test is treated as a new test, not a retest, so it costs the full price. If the second station gives you a different result, contact the Bureau of Automotive Repair to report the discrepancy.