What CO emissions testing measures

CO emissions testing measures the amount of carbon monoxide your vehicle produces during operation. Carbon monoxide is a colorless, odorless gas created when fuel burns incompletely in your engine. The test draws exhaust from your tailpipe and analyzes it using an infrared sensor or similar device, reporting the concentration in parts per million (ppm).

Most states that conduct emissions testing include CO as part of their standard test. The specific limits vary by state, vehicle year, and whether your car uses a catalytic converter. Older vehicles typically have higher CO limits than newer ones because emission control technology has improved over decades. A vehicle that passes CO testing has a functioning fuel system and ignition timing that allows most fuel to burn completely.

The test itself takes only a few minutes. A technician connects a probe to your exhaust pipe, runs the engine at idle and sometimes at a higher RPM, and records the readings. You receive a printout showing whether you passed or failed, and if you failed, which pollutants exceeded the limit.

Key Takeaways

  • CO emissions testing measures carbon monoxide in your exhaust and is part of most state inspection programs, though not all states require it.
  • Failing a CO test usually points to problems with your fuel injectors, oxygen sensor, spark plugs, or catalytic converter, not necessarily a major engine failure.
  • The cost of a CO emissions test ranges from $20 to $50 depending on your state and testing facility, and is separate from your vehicle inspection fee.
  • If you fail, you can have repairs made and retested at the same facility, and most states allow a set number of free or discounted retests within a certain timeframe.

Why states test for CO emissions

States include CO testing in their emissions programs because carbon monoxide is a public health hazard. When inhaled in high concentrations, CO binds to hemoglobin in your blood and reduces oxygen delivery to your organs and brain. People with heart disease, children, and pregnant women are especially vulnerable to CO exposure.

The Environmental Protection Agency (EPA) sets national standards for CO emissions, and states design their testing programs to enforce those standards. By requiring regular testing, states can identify vehicles that are malfunctioning and remove them from the road until they are repaired. This approach has reduced overall CO pollution in urban areas significantly since emissions testing began in the 1970s.

Not every state requires CO testing. Some states have moved to OBD (on-board diagnostic) testing instead, which reads fault codes directly from your vehicle's computer rather than sampling exhaust. Other states have eliminated emissions testing altogether for newer vehicles or in counties that meet federal air quality standards. Check your state's Department of Motor Vehicles or environmental agency website to learn whether CO testing is required where you live.

Common reasons for CO test failures

A high CO reading usually indicates that your engine is not burning fuel completely. The most common causes are a faulty oxygen sensor, which tells your engine how much fuel to inject; fouled or worn spark plugs, which ignite the fuel; or a clogged fuel injector, which sprays fuel into the cylinder. All three are relatively inexpensive repairs, typically ranging from $100 to $400 depending on your vehicle.

A failing catalytic converter can also cause high CO. The catalytic converter is a component in your exhaust system that chemically converts CO into carbon dioxide. If it is damaged or clogged, CO will pass through to the tailpipe unfiltered. Catalytic converter replacement is more expensive, usually $500 to $2,500, but is often covered under your vehicle's warranty if your car is still under the manufacturer's coverage period.

Less commonly, a CO failure can signal problems with your engine's air intake, fuel pressure regulator, or ignition timing. A technician at a repair shop can run a diagnostic scan to pinpoint the exact cause. Many shops offer free or low-cost diagnostics if you mention you failed an emissions test.

What happens after you fail a CO test

If your vehicle fails the CO test, you will receive a report showing the exact reading and the limit your vehicle must meet. You then have a set window—usually 30 to 60 days depending on your state—to have repairs made and return for a retest. Some states allow you to retest at any certified facility; others require you to return to the same location.

Most states do not charge a full retest fee if you return within the allowed timeframe. Some offer one free retest; others charge a reduced fee of $10 to $20. If you fail a second time, you may be required to pay the full test fee again and may face additional restrictions, such as a requirement to have a mechanic's report documenting the repairs attempted.

If you do not retest within the allowed period, your vehicle registration may be suspended or you may be unable to renew it. Some states allow you to request an extension if you can show proof that repairs are in progress or that you are waiting for parts. Contact your state's emissions program or your local testing facility to understand the exact timeline and extension process in your area.

How CO test limits are set

CO limits are determined by your vehicle's model year and engine type. The EPA sets national baseline standards, but states can adopt those standards or set their own, which are often stricter. A vehicle from 2000 or newer typically has a CO limit of 0.3% at idle, while older vehicles may have limits of 1% or higher.

Some states use a two-speed test, measuring CO at both idle and at 2,500 RPM. The higher RPM reading is often more lenient because engines naturally produce less CO when running at higher speeds. Other states test only at idle. A few states have transitioned entirely to OBD testing, which does not measure CO directly but instead checks whether your vehicle's emission control systems are functioning.

If you are unsure what the CO limit is for your vehicle, ask the technician before the test. They can tell you the exact threshold your car must meet and explain what the reading means if you fail.

Preparing your vehicle for a CO test

Before scheduling a CO test, make sure your vehicle has been driven for at least 20 minutes so the engine reaches normal operating temperature. A cold engine can produce artificially high CO readings. If your check engine light is on, have the vehicle scanned for fault codes before testing, as some codes will cause an automatic test failure regardless of your CO reading.

Keep your maintenance records current. Vehicles with regular oil changes, fresh spark plugs, and a clean air filter tend to produce lower CO emissions. If you know your vehicle has been sitting unused for weeks, run it for a while before testing to clear out any buildup in the fuel system.

Do not attempt to mask a high CO reading by adding fuel additives or using octane boosters. These do not address the underlying problem and will not lower your CO emissions enough to pass. If your vehicle is failing CO tests repeatedly, the issue is mechanical and requires actual repair.

CO testing versus other emissions tests

CO is one of several pollutants measured in emissions testing. Depending on your state and vehicle, you may also be tested for nitrogen oxides (NOx), hydrocarbons (HC), and particulate matter (PM). Each pollutant has its own limit and points to different mechanical problems.

NOx is produced when fuel burns at very high temperatures and is controlled by your vehicle's exhaust gas recirculation (EGR) system. HC is unburned fuel and indicates problems similar to those that cause high CO. PM, measured mainly in diesel vehicles, is soot and ash in the exhaust.

Some states have moved away from tailpipe testing altogether and now use OBD testing, which reads your vehicle's onboard computer to check whether emission control systems are working. OBD testing is faster and does not require a probe in the exhaust, but it does not directly measure pollutant levels. If your state uses OBD testing, you will not receive a CO reading, but your vehicle must still pass a visual inspection of emission-related components.

Frequently Asked Questions

Can a new car fail a CO emissions test?

Yes, though it is rare. New vehicles have sophisticated emission control systems, but a manufacturing defect, a recent accident, or a faulty sensor can cause a CO failure. If your vehicle is under warranty and fails, contact the dealership—they can often diagnose and repair the problem at no cost to you.

What does it mean if my CO reading is just slightly over the limit?

A reading just above the limit usually points to a minor issue like a worn spark plug or a slightly dirty fuel injector. These are inexpensive repairs. However, some states do not allow partial passes—you either pass or fail—so even a small overage requires a retest after repairs.

How long does a CO emissions test take?

The actual test takes 5 to 10 minutes. Waiting time at the testing facility depends on how busy they are and can range from a few minutes to an hour. Call ahead to ask about wait times, or check whether your state allows online scheduling.

If I fail a CO test, do I have to go back to the same testing facility?

Most states allow you to retest at any certified facility, but a few require you to return to the original location. Check your test report or contact your state's emissions program to confirm the rule in your area.

Can I dispute a CO test result?

Yes. If you believe the test was conducted incorrectly or the equipment was faulty, you can request a retest at a different facility. Some states allow you to challenge the result within a set timeframe. Contact your state's emissions program or the testing facility for the formal dispute process.