What the emissions warning means
The check engine light, formally called the malfunction indicator lamp, is closely tied to emissions. Under OBD-II rules, the car’s computer continuously and periodically tests the parts that control exhaust and fuel vapor. When it detects a fault that could raise emissions, it stores a code and turns on the light. Some vehicles also display a message such as “Emission Control System Malfunction” or “Service Emissions System.” Wording varies by manufacturer, but the meaning is generally the same: the computer has found a problem in a system that affects emissions.
The warning does not by itself tell you which part is at fault. To find out, the stored code needs to be read. Our article on reading check engine codes explains how.
Parts the emissions system monitors
- Catalytic converter. Converts harmful exhaust gases into less harmful ones. Efficiency codes such as P0420 are common.
- Oxygen (air-fuel ratio) sensors. Measure exhaust content so the computer can adjust fueling. Aging sensors are a frequent source of codes.
- EVAP system. Captures fuel vapor from the tank. Gas cap issues, cracked hoses, and faulty valves can set codes.
- EGR system. Recirculates some exhaust back into the engine on many vehicles, and can clog or stick.
- Ignition and fuel components. Misfires from worn spark plugs or coils raise emissions and can set codes.
- Air intake sensors. A faulty mass airflow sensor can throw off the air-fuel mixture.
- Secondary air injection and PCV systems. Present on some vehicles and can also set codes.
Common causes, from simplest to costliest
- Gas cap. A loose, cracked, or missing cap is a commonly cited cause of small EVAP leak codes. Tighten it until it clicks. The light may take some driving to turn off.
- Spark plugs and ignition coils. Misfires are a common reason for emissions-related codes.
- Oxygen sensors or air-fuel sensors.
- EVAP hoses and valves.
- Mass airflow sensor or vacuum leaks.
- Catalytic converter. This tends to be the most expensive repair, and it is wise to rule out other causes before replacing it, since misfires, bad sensors, or exhaust leaks can mimic a failing converter. Our article on whether a muffler hole can turn on the check engine light discusses one such exhaust-related possibility.
Can I drive with the emissions light on?
If the light is steady and the car runs normally, many drivers are able to continue driving for a short time while arranging a diagnosis. If it flashes, or the car hesitates, stalls, or overheats, it is safer to stop driving and get help, since a severe misfire can damage the catalytic converter. Ignoring a steady light for long periods can allow some problems to grow and may lead to higher repair costs.
Can the car pass an emissions test with the light on?
In most places that test OBD-II vehicles, an illuminated check engine light is typically an automatic failure. Also, if codes were cleared recently, the car’s readiness monitors may be incomplete, and some programs will reject the vehicle until they are set. Requirements vary by state and locality, so check your local program. We cover this more in why cars fail emissions tests and how to fix it.
Steps to take
- Note any message on the dashboard and whether the light is steady or flashing.
- Check the gas cap.
- Get the codes read, either with your own scanner or at a parts store or shop.
- Research the code for your vehicle, then fix the likely cause.
- Drive normally for several trips so the car can rerun its self-tests. The light may turn off by itself if the fault is gone.
- If the light returns, ask a technician for a proper diagnosis rather than replacing parts by guess.
Emissions warranty
In the United States, federal rules require manufacturers to cover certain emissions parts for a set period. Coverage details differ by part and vehicle, and sometimes by state, so check your warranty documents or ask a dealer before paying for an emissions repair on a newer car.
Frequently asked questions
What does the emission control system malfunction light mean?
It means the computer detected a fault in a part that affects emissions. The specific cause requires reading the code.
Why does a brand-new warning appear after refueling?
A loose or incorrectly closed gas cap is a commonly cited reason. Re-tighten it and see whether the light turns off after some driving.
How much does it cost to fix exhaust emissions problems?
It varies widely. A gas cap is inexpensive, while a catalytic converter can cost significantly more. A written estimate after proper diagnosis is the best way to know your number.