What a smog check tests and why your car might fail
A smog check measures whether your vehicle's emissions fall within the limits set by your state's air quality standards. The test itself is straightforward: a technician connects a probe to your vehicle's tailpipe and reads the levels of nitrogen oxides, carbon monoxide, and other pollutants your engine produces. The standards vary by state and sometimes by county, and they also depend on your vehicle's model year and engine type.
Most vehicles fail because of one of three problems: a faulty oxygen sensor, a malfunctioning catalytic converter, or an engine control issue that triggers the check engine light. Some fail because the gas cap is loose or missing—a surprisingly common reason. Others fail because the vehicle hasn't been driven enough since the last repair to clear old fault codes from the computer. A few fail because the engine is straightforward running too rich (burning too much fuel relative to air), which usually points to a fuel injector problem or a bad mass airflow sensor.
Your state's smog check program will tell you whether your vehicle is subject to testing. Generally, vehicles older than a certain age (often 1975 or earlier) are exempt, as are some newer vehicles depending on state rules. Hybrid and electric vehicles are often exempt as well. Check your state's air quality agency website or your local smog station to confirm whether your vehicle needs to be tested.
Key Takeaways
- A smog check measures tailpipe emissions against your state's air quality standards, and most failures come from a faulty oxygen sensor, bad catalytic converter, or check engine light.
- Before you test, make sure your gas cap is tight, your tires are properly inflated, and your engine oil is at the correct level—all three affect emissions readings.
- If your check engine light is on, get the code read at an auto parts store or mechanic before you pay for a smog test, because you will fail if the light is illuminated.
- A failed test does not mean your vehicle is unsafe to drive; it means repairs are needed before you can renew your registration in most states.
- Some states allow a second test within a set window at no additional cost if you fail the first time, so ask the smog station about their retest policy.
Preparing your vehicle before the test
The single most important step is to make sure your check engine light is not on. If it is, the smog station will fail you when ready without even running the emissions test. Take your vehicle to an auto parts store—most offer free diagnostic code reading—or to a mechanic and have the code pulled. Common codes that trigger the light include P0420 (catalytic converter efficiency), P0133 (oxygen sensor circuit), and P0171 (system too lean). Once you know the code, you can decide whether to repair it before testing or to address it after a failed test.
Next, check three things that directly affect emissions readings: tire pressure, engine oil level, and the fuel cap. Underinflated tires increase rolling resistance and make your engine work harder, which raises emissions. Check your vehicle's door jamb or owner's manual for the correct pressure and inflate to that number, not to the maximum printed on the tire itself. Low engine oil thickens and burns less efficiently, so top it off if needed. A loose or missing gas cap allows fuel vapors to escape, which will cause a failure on the evaporative emissions portion of the test.
If your vehicle has been sitting for weeks or if you have only driven it short distances since the last repair, take it on a 15- to 20-minute highway drive at steady speed before your test. This allows the engine to reach full operating temperature and helps clear temporary fault codes from the computer. Do not do this if your check engine light is on, because you want the technician to see the actual problem.
Understanding the two types of smog tests
Most states use one of two testing methods: the OBD-II test or the tailpipe test. The OBD-II test (on-board diagnostics) is used for vehicles from 1996 onward in most states. The technician plugs a scanner into the diagnostic port under your dashboard, reads your vehicle's onboard computer, and checks whether the emissions system is functioning correctly and whether any fault codes are stored. This test does not measure actual emissions; it checks whether your vehicle's computer says the system is working. If your check engine light is on or if the computer shows a fault, you fail.
The tailpipe test is used for older vehicles (typically 1995 and earlier) and in some states for all vehicles. The technician inserts a probe into your tailpipe and measures the actual gases coming out. The readings are compared against your state's standards for your vehicle's model year and engine size. This test is more sensitive to engine condition and maintenance, so a vehicle that runs rough or has not been tuned recently is more likely to fail.
A few states use a hybrid approach: they run the OBD-II test first, and if the vehicle passes, they do not run the tailpipe test. If the OBD-II test shows a fault, they may require the tailpipe test as well. Ask your smog station which method they will use before you arrive, so you know what to expect.
What happens if you fail the test
A failed test does not mean your vehicle is unsafe or that you have broken the law. It means your vehicle's emissions are above the legal limit for your state, and you cannot renew your registration until you repair the problem and pass a retest. The smog station will give you a report showing which emissions were too high and which systems may be at fault.
Take that report to a mechanic and discuss the likely causes. If the report shows high nitrogen oxides, the oxygen sensor or catalytic converter is the most common culprit. If it shows high carbon monoxide, the fuel system or ignition timing may be the problem. A good mechanic can often narrow down the issue based on the specific readings and the fault codes (if any) stored in your vehicle's computer.
Some states allow a free retest within 30 days if you fail, while others charge a reduced fee. A few states allow you to retest at any station, while others require you to return to the same station. Check your state's rules or ask the smog station about their retest policy before you leave. If you repair the vehicle and pass the retest, you can then renew your registration.
Common repairs that help you pass
The most common repair is replacing the oxygen sensor, which typically costs between $150 and $400 depending on the vehicle and whether it is the upstream or downstream sensor. The oxygen sensor tells your engine how much fuel to burn, and a faulty sensor causes the engine to run too rich or too lean, both of which increase emissions. If your smog report shows high carbon monoxide or high hydrocarbons, this is often the first thing to replace.
Catalytic converter replacement is more expensive—usually $400 to $1,200 or more—but is necessary if the converter is clogged or damaged. A failing catalytic converter will show up as high nitrogen oxides on the tailpipe test or as a P0420 fault code on the OBD-II test. Some states offer repair information programs for low-income vehicle owners facing high repair costs; check your state's air quality agency website to see whether you may be may be able to access.
Smaller repairs that sometimes help include replacing a faulty fuel cap (usually under $20), cleaning the fuel injectors (typically $50 to $100), and replacing spark plugs if they are old (usually $100 to $300). A mechanic can advise you on which repairs are most likely to solve your specific emissions problem based on your smog report and any fault codes.
Choosing a smog station and scheduling your test
Smog stations are licensed by your state's air quality agency, and you can find a list of certified stations on your state's website or through a straightforward search. All certified stations use the same testing equipment and follow the same standards, so the results should be consistent regardless of which station you choose. However, some stations may have shorter wait times or more convenient hours.
Call ahead or check online to see whether the station requires an appointment or accepts walk-ins. Bring your vehicle's registration, proof of insurance, and a valid photo ID. The test itself usually takes 10 to 15 minutes. The fee varies by state but typically ranges from $30 to $70 for a single test. Some stations offer package deals if you need a retest.
Schedule your test during a time when your vehicle has been driven recently and is in good condition. Avoid scheduling when ready after a long period of sitting or after you have noticed rough running or warning lights. If you have just had repairs done, wait a few days and drive the vehicle normally before testing, so the computer can verify that the repairs worked.
What to do if you cannot afford repairs
Several states offer repair information or vehicle replacement programs for low-income drivers whose vehicles fail smog checks. California's Consumer information Program, for example, can help pay for repairs or provide a replacement vehicle if repairs would cost more than a certain percentage of the vehicle's value. Texas, Colorado, and other states have similar programs. Check your state's air quality agency website or call your local air quality district to ask whether information is available in your area.
Some nonprofits and community colleges also offer low-cost repair services. A search for "low-cost auto repair" or "community college auto shop" in your area may turn up options. If you cannot afford repairs and no information program is available, you may be able to defer registration renewal in some states, though this is usually temporary and you will eventually need to address the emissions problem to keep your vehicle legal.
Frequently Asked Questions
Can I pass a smog check if my check engine light is on?
No. If your check engine light is illuminated, you will fail the test when ready. The light indicates that your vehicle's onboard computer has detected a fault in the emissions system. Have the code read before you test, and either repair the problem or understand what it is so you can plan repairs after the test.
How long does a smog check take?
The actual test takes 10 to 15 minutes. If you are waiting in line at a busy station, the total time may be 30 minutes to an hour. Calling ahead to ask about wait times or scheduling an appointment can help you avoid a long wait.
Do I need to pass a smog check every year?
It depends on your state and your vehicle's age. Most states require smog checks every two years for vehicles over a certain age (often 1975 or later). Some states require annual checks for older vehicles. Check your state's air quality agency website or your vehicle's registration documents to find out your specific requirement.
What if I fail the retest after repairs?
If you fail again, the repairs did not solve the problem. Return to your mechanic with the second smog report and discuss what else might be causing the high emissions. It is possible that multiple systems need repair, or that the initial diagnosis was incomplete. Some mechanics offer a warranty on repairs, so ask whether yours will re-examine the vehicle at no charge.
Can I drive my vehicle if it fails a smog check?
Yes, you can drive it. A failed smog check does not make your vehicle unsafe or illegal to operate. However, you cannot renew your vehicle registration until you pass the test, so you cannot legally register it in your state. Plan your repairs before your registration expires.