What CO emissions testing checks for

A CO emissions test measures how much carbon monoxide your vehicle's engine produces during operation. The test runs your car on a dynamometer (a machine that simulates driving) or uses a probe inserted into your tailpipe while the engine idles. The machine records the concentration of CO in your exhaust in parts per million (ppm).

Carbon monoxide is a colorless, odorless gas created when fuel burns incompletely in your engine. High CO levels mean your engine is not burning fuel efficiently, which also wastes gas and can damage your catalytic converter. States that require CO testing do so because excessive emissions contribute to air pollution and smog, particularly in urban areas and regions with poor air quality.

Most states that still perform CO testing use one of two methods: a loaded test, where your car runs under simulated driving conditions, or an idle test, where the engine runs at a standstill. The specific method and pass/fail threshold depend on your state's rules and your vehicle's model year.

Key Takeaways

  • CO testing measures carbon monoxide in your exhaust; high levels indicate incomplete fuel burning and engine problems.
  • Only about half of U.S. states still require CO testing as part of vehicle inspections, and many have phased it out in favor of OBD-II scanning.
  • Your vehicle's age, fuel type, and whether it has a catalytic converter all affect whether it must pass a CO test.
  • If your car fails a CO test, common fixes include replacing spark plugs, cleaning fuel injectors, or repairing the catalytic converter.
  • You can find out whether your state requires CO testing by contacting your local emissions testing facility or checking your state's environmental agency website.

Which states still require CO testing

CO testing requirements have shrunk significantly over the past two decades. Many states have replaced CO testing with OBD-II scanning, a method that reads diagnostic codes directly from your vehicle's onboard computer. OBD-II is faster, requires no tailpipe probe, and works on all vehicles from 1996 onward.

States that still perform CO testing typically do so in areas with documented air quality problems or as part of a combined emissions inspection that also includes OBD-II scanning. The specific rules vary widely: some states test all vehicles, others only those above a certain age, and some exempt newer cars or vehicles with low mileage.

Your state's environmental agency or your local inspection station can tell you whether CO testing is required where you live. If you are moving to a new state or buying a vehicle registered elsewhere, check the rules in your destination state before your inspection is due.

What happens if your car fails a CO test

A failed CO test means your engine is producing more carbon monoxide than your state's limit allows. The test report will show your vehicle's CO reading and the maximum allowed for your vehicle's year and type. You then have a set window—usually 30 to 60 days—to repair the vehicle and retest.

Common causes of high CO emissions include a faulty oxygen sensor, worn spark plugs, a clogged fuel filter, dirty fuel injectors, or a failing catalytic converter. A mechanic can run a diagnostic scan to pinpoint the problem. Some repairs are inexpensive (spark plugs, air filter), while others (catalytic converter replacement) can cost several hundred dollars.

If you repair the vehicle and retest within the allowed timeframe, you pay only the retest fee, which is typically $10 to $30. If you miss the important date, you may have to pay the full inspection fee again or face registration penalties depending on your state.

How CO testing differs from OBD-II scanning

OBD-II scanning reads error codes stored in your vehicle's computer without measuring actual tailpipe emissions. It works on all 1996 and newer vehicles and takes about five minutes. CO testing, by contrast, physically measures exhaust gases and requires your engine to run under specific conditions.

OBD-II is faster and less invasive, but it only detects problems your vehicle's computer has already flagged. CO testing can sometimes catch emissions issues before they trigger a check-engine light. However, OBD-II is also more standardized across states, whereas CO testing thresholds and procedures vary.

Many states now use a hybrid approach: they perform OBD-II scanning on all vehicles, then add CO testing only for those that fail the computer scan or for vehicles in specific age ranges. Ask your inspection station which method applies to your vehicle.

Preparing your vehicle for a CO test

Before your CO test, make sure your vehicle is in basic working order. Change your oil and air filter if they are due, may support your spark plugs are not excessively worn, and have your fuel system cleaned if you notice rough idling or poor acceleration. A well-maintained engine is far more likely to pass.

Warm up your engine for a few minutes before the test—a cold engine produces higher CO levels. If your vehicle has been sitting unused for weeks, drive it for 10 to 15 minutes before heading to the inspection station. Avoid heavy acceleration or driving at high speeds when ready before testing, as this can skew results.

If your vehicle has a check-engine light on, address that before testing. Many states will not perform a CO test on a vehicle with an active warning light, and even if they do, the underlying problem is likely to cause a failed test.

What to do if your state no longer requires CO testing

If your state has phased out CO testing in favor of OBD-II scanning, your inspection will be simpler and faster. You will not need a tailpipe probe or a dynamometer run. However, you still need to pass the OBD-II scan, which means your vehicle's computer cannot have unresolved diagnostic trouble codes.

If your state has eliminated emissions testing altogether, you may only need a basic safety inspection (lights, brakes, windshield wipers, horn). Check your state's Department of Motor Vehicles or environmental agency website to confirm what your vehicle must pass.

Some states allow you to check whether your vehicle will pass an OBD-II scan before you officially test. Many inspection stations offer a pre-inspection scan for a small fee, which can save you time and money if problems are found early.

Frequently Asked Questions

Can I pass a CO test if my check-engine light is on?

Most states will not perform a CO test if your check-engine light is active, and even if they do, the underlying problem will likely cause a failure. Have the code read and the issue repaired before testing. Many auto parts stores read codes for free.

How much does a CO emissions test cost?

CO testing fees vary by state and facility, typically ranging from $15 to $50 for the initial test. Retests after repairs are usually cheaper, around $10 to $30. Some states bundle CO testing with OBD-II scanning into a single inspection fee.

What is the difference between CO and NOx testing?

CO (carbon monoxide) measures incomplete fuel burning. NOx (nitrogen oxides) measures pollutants created by high combustion temperatures. Most states that still test emissions focus on CO and OBD-II; NOx testing is less common and usually only required for diesel vehicles or in areas with severe air quality problems.

Do electric vehicles have to pass CO testing?

No. Electric vehicles produce zero tailpipe emissions and are exempt from CO and OBD-II testing in all states. Plug-in hybrids may be subject to testing depending on your state's rules—check with your local inspection station.

What if I fail a CO test twice?

If you fail twice, your state may allow you to explore for a waiver or extended important date, or you may be required to retire the vehicle from road use. Rules vary significantly by state. Contact your state's environmental agency or your inspection station for options if you are facing a second failure.