The inclusion of a bright light on your dash that says to check your emission system seems like the perfect warning for those oblivious to the steam locomotive-like trail of smoke behind them. Ask us how we know that.
However, maybe that’s a bit overkill, as an emission system light would already warn you beforehand of many other kinds of issues that may not be as noticeable as a cloud in your rearview.
So before your car ends up casting for “Thomas and Friends”, let’s have you know what a check emission system light actually means and how to fix it!
What does a check emission system light mean?
A check emission system warning light or message means that there is a potential problem detected in your vehicle’s emission system.
In addition, a check emission system light can also mean that the ECU has detected a problem with the fuel system or ignition system.
If you ever get a check emission system light or message on your dashboard or instrument panel, then most of the time, the message already narrows it to a problem with a component of the emission control system.
Depending on the vehicle model, the check emission system light may be in the form of a small engine icon similar to that of a check engine light (CEL).
However, like any other system, the emission control system has several different components that all work together to ensure that toxic pollutants that come out of your exhaust are kept to a minimum.
Hence, even if one of these emission components were to fail, then the check emission system light would illuminate and the performance of your vehicle may be reduced to prevent excessive emissions.
Though the list of reasons doesn’t stop there, as a check emission system light can also be triggered by a faulty component in either the fuel system or ignition system.
It makes perfect sense, as the fuel system and ignition system work hand-in-hand to burn the air-fuel mixture in the engine to create power, which eventually results in the byproduct of exhaust emissions.
So what exactly are these list of components that commonly cause a check emission system light to appear? Let’s first have a general summary below.
What causes the check emission system light to appear?
A check emission system light is commonly caused by a faulty oxygen sensor, a faulty MAF sensor, a bad catalytic converter, and other emission system issues.
A check emission system light can also be caused by a faulty fuel injector, bad spark plugs and ignition coils, and a loose or damaged gas cap.
As you can see by our summarized answer above, there are quite a lot of possible reasons why you’d get a check emission system light on any vehicle (save for EVs, obviously).
But to further understand why the failure of such components can give you the unpleasant surprise of a check emission system light or message, we’ll be going over each of them below.
Faulty Oxygen (O2) Sensor
Whenever we get reports that a check emission system light or notification appears on someone’s dashboard, one of the most common culprits to this is one of the oxygen (O2) sensors becoming faulty.
There are several of these O2 sensors installed at certain points of a vehicle’s exhaust system, and they’re all tasked to monitor the level of unburnt oxygen present in your emissions.
Whatever readings the O2 sensors get will be sent to the vehicle’s ECU (engine/electronic control unit) to gauge if the engine has the correct air-fuel ratio and is burning fuel efficiently.
Going by this, if one of the O2 sensors malfunction and give off oxygen level readings that are either too high or too low, the ECU will compensate by adjusting the air-fuel ratio inside the engine.
This unnecessary adjustment in the air-fuel ratio can result in increased emissions and the check emission system light turning on.
Faulty Mass Airflow (MAF) Sensor
Another sensor that can trigger the check emission system light or notification when faulty is the mass airflow sensor or “MAF” sensor.
Similar to an O2 sensor, the MAF sensor measures the amount of air entering the engine via the air intake. The gathered data will also be sent to the ECU to gauge if it’s the correct amount of air.
Thus, if a MAF sensor becomes faulty, abnormal readings on the amount of air will be sent to the ECU, which can then cause your vehicle to consume too much fuel and emit more exhaust gases.
And of course, the emission of more exhaust gases means there is a possibility that the check emission system light may also get triggered.
Bad Catalytic Converter
Your vehicle’s catalytic converter is an important emission system component that converts toxic byproducts of the engine’s combustion process into less harmful emissions.
However, once something goes wrong with the catalytic converter, chances are that there are other underlying problems with your other emission system components you’re unaware of.
For instance, catalytic converters are known to go bad whenever there are any issues with the O2 sensors that have not been taken care of right away.
Since we already know that faulty O2 sensors can lead to more emissions, they can also indirectly cause damage and prematurely wear the catalytic converter due to the increased load.
Once the catalytic converter gets clogged or completely fails, then the check emission system light can also turn on.
Exhaust Leaks
If your vehicle has suddenly given you a check emission system light or notification, then it’s also worth checking for any leaks in your exhaust system.
One specific component to check for exhaust leaks is none other than the exhaust manifold, which is the first part of the exhaust system that directly collects gases from each of the engine’s cylinders.
Aside from that, you should also check for any cracks or loose clamps in any of the exhaust pipes that run underneath your vehicle, as these can also be a potential source of an exhaust leak.
As we’ve mentioned earlier, your vehicle’s exhaust system contains multiple O2 sensors monitoring the flow of oxygen.
So if there’s an exhaust leak somewhere, one of the O2 sensors may give off unusual readings to the ECU, which will then trigger the check emission system warning light or message.
Faulty EVAP System
The EVAP system, short for “Evaporative Emission Control System”, is designed to prevent vapor from the fuel system from leaking into the surrounding atmosphere.
However, once the EVAP system fails, it can do the exact opposite of what it’s intended to do. If it leaks fuel vapor, not only is your vehicle creating more harmful emissions, but it’s also a potential source of a fire hazard.
If you’ve confirmed that the check emission system light is caused by a problem with your EVAP system, then you should waste no time at all getting your vehicle checked out for any repairs or replacements.
Faulty Exhaust Gas Recirculation (EGR) Valve
Continuing the trend of leak-related problems that can trigger the check emission system light, you should also inspect your EGR valve for any faults or damages.
The EGR valve, or “exhaust gas recirculation” valve, is another important component of the emission control system that recirculates a certain amount of exhaust gases back into the intake system to reduce emissions.
The EGR system primarily controls NOx (nitrogen oxide) emissions by lowering oxygen levels in the combustion chamber, so a malfunction in the EGR valve can cause an increase in NOx being emitted from the engine.
Not only can increased NOx emissions trigger the check emission system light, but it’s also accompanied by higher combustion temperatures, in which the resulting hotter gases will eventually pass through your catalytic converter and wear it faster.
Clogged Diesel Particulate Filter
Diesel vehicles of today are fitted with a type of emission control device called a “diesel particulate filter”, and as its name suggests, it’s tasked to reduce the amount of diesel particulate matter or “soot” from exhaust emissions.
Due to the nature of its job, the diesel particulate filter can potentially become clogged because of too much soot buildup.
Once the diesel particulate filter gets clogged or fails, you’ll be getting increased diesel emissions and you may feel a drop in the engine’s performance.
Depending on the specific diesel vehicle, a clogged or failed diesel particulate filter may trigger a check emission system light or a separate “DPF” warning light on the dashboard.
Loose or Damaged Gas Cap
One reason that you may not have expected can cause the check emission system light to appear is a loose or damaged gas cap.
While the gas cap isn’t exactly part of the emission control system, it is a potential source of fuel vapor leaks if you were to accidentally leave it even just a bit open after your last refueling stop.
This also applies if the gas cap was to have some form of damage or crack on it, which would allow fuel vapor to leak out of the fuel tank and into the surrounding area.
Similar to a damaged EVAP system that we talked about earlier, a loose or damaged gas cap can not only trigger the check emission system light but also a potential fire hazard due to the escaping of fuel vapor into the air.
So for this reason, make sure to always properly close your gas cap every time you fill up for gas. And if you notice any signs of damage on it, have it replaced right away.
Dirty Air Filter
The air filter on your vehicle serves as the first line of defense against dirt, dust, insects, or any foreign particles that may end up inside your engine.
If your air filter becomes too dirty or gets clogged, it can significantly restrict the amount of air that is supposed to enter the engine, affecting the delicate air-fuel ratio that’s needed for an optimum and efficient combustion process.
With a dirty or clogged air filter, the reduced airflow will make the engine run rich (too much fuel and not enough air), which means that emissions will also increase and thus trigger the check emission system light.
So if you’re experiencing decreased engine performance and poorer fuel economy, you may want to check your air filter for the presence of any creepy crawlies and whatnot that are choking the engine, then clean or replace the filter as needed.
Failed Fuel Injector
The presence of a check emission system light or notification on your dashboard doesn’t necessarily limit the problem to emission components alone, as issues with the fuel system can also be possible culprits.
One such culprit is a bad or failed fuel injector, which can either inject too much or too little fuel inside the combustion chamber, and we already know what happens when the optimum air-fuel mixture gets thrown out the window.
A failed fuel injector can cause either incomplete or uneven burning of fuel inside the engine, which eventually leads to increased emissions out of your vehicle’s exhaust and the check emission system light flashing once again on your dash.
Faulty Spark Plugs and Ignition Coils
Other parts that are worth checking for issues if you’re experiencing a check emission system light are the spark plugs and ignition coils.
Despite being part of the ignition system instead of the emission control system, once a fault develops in either the spark plugs or coils, the combustion process will be affected and increased emissions can occur.
Not only can bad spark plugs and ignition coils result in a check emission system light, but it’s also commonly known to cause misfires (misfire after changing spark plugs) that can trigger the check engine light (CEL) as well.
Vacuum Leaks (Damaged Vacuum Hose)
We’re not quite done talking about leaks just yet, as vacuum leaks can also be a potential culprit to the check emission system light or message popping up on your vehicle.
Vacuum leaks can occur due to damaged vacuum hoses that are part of the intake system. If the hoses are loose, then that can also be a source of a vacuum leak.
Whenever there’s a vacuum leak, you bet that the air-fuel ratio of the engine will become affected again, which can increase emissions and cause the check emission system light or even the check engine light (CEL) to appear.
Diagnosing a Check Emission System Warning Light
If you get a check emission system warning light, there may be error codes stored in the vehicle’s ECU that would be essential in diagnosing the cause.
You can use a scan tool like an OBD-II reader or have a mechanic diagnose your car to find out what’s causing the check emission system light.
How do you fix and reset a check emission system light?
A check emission system light can be fixed by replacing the faulty component that’s causing it to appear, like a bad oxygen sensor or MAF sensor.
To reset and clear a check emission system light, you can either use a scan tool or disconnect the battery for at least 30 seconds.
After figuring out what’s causing the check emission system light to come on, you can then check the faulty components to see if they can still be repaired or are already due for replacement.
For instance, if the ECU has stored an error code (e.g. P0138) that says that an oxygen (O2) sensor has stopped working, then you would need to replace that O2 sensor right away.
Now how about actually making the check emission system light go away on your dash? Well, there are two main ways to go about it, and the first one is to simply use the same scanner to clear the error code and make the light disappear.
The second way to reset the check emission system light is to disconnect the battery for at least 30 seconds, which will reset the ECU itself and cause the light to turn off.
Is it safe to drive with a check emission system light turned on?
It is not safe to drive while a check emission system light or message is present, as there can be many different reasons for it that can worsen and lead to more severe damages and dangerous situations.
Whether the cause is a faulty O2 sensor, a dirty air filter, or an open gas cap, it’s always recommended to find the reason why a check emission system light or notification has appeared before driving your car any further.
Due to the many possible reasons that can trigger a check emission system light, it can be difficult to tell how serious the problem is unless you get your vehicle diagnosed by a mechanic.
Thus, you should treat a check emission system light as seriously as any other kind of warning on your car that can affect its ability to run properly.
How to Prevent the Check Emission System Light From Turning On
You can prevent the check emission system light from turning on by always doing regular maintenance on your car, filling it up with only high-quality or recommended fuel, and making sure your gas cap is properly secured.
Regular Maintenance
Performing regular maintenance on your vehicle is one of the best things that you can do to prevent not just a check emission system light, but a myriad of other issues.
Whether you’re doing it yourself or having the dealership or local mechanic do the dirty work for you, it’s always important to stick to the recommended service intervals for your vehicle’s wear-and-tear components and fluids.
This can mean doing regular oil changes and cleaning or replacing any dirty filters so that your emission system is able to do its job optimally.
Using Recommended/High-Quality Fuel
Another good tip to prevent your check emission system light from illuminating is to use only high-quality fuel or the recommended fuel type for your vehicle.
Recommended fuel types are indicated in your owner’s manual, which can include info like the minimum octane rating, ethanol compatibility, and additives or materials you should avoid using.
Using high-quality fuel or recommended fuel will allow for a cleaner and more complete combustion process, which will prevent your emission system from excessive wear and damage.
Properly Securing the Gas Cap
After you’ve filled up your car with high-quality fuel, it’s also important to remember to properly secure your gas cap so as to not cause any fuel or vapor to leak out.
Leaving a gas cap even just slightly loose can trigger the check emission system light and also become a potential fire hazard, so be sure to close it tightly every single time.
Is my gas mileage affected while the check emission system light is on?
A check emission system light can potentially affect the gas mileage of your vehicle due to the many causes associated with it.
Problems like bad O2 sensors or faulty catalytic converters are known to cause both a check emission system light and poorer gas mileage.
How much does it cost to fix and turn off a check emission system light?
The total cost to fix and turn off a check emission system warning light depends on what’s causing it to appear in the first place.
If the cause is a faulty O2 sensor, then you’ll be spending about $100 for a new one. Otherwise, faulty emission components can cost over $200 to replace.
Conclusion
We’ve learned that a check emission system light or message can be caused by many different issues in either your emission system, fuel system, or ignition system.
The causes can vary from faulty sensors and loose gas caps to EVAP system issues and a bad catalytic converter, which means total repair costs can vary a lot as well.
Thus, it’s important to have your vehicle diagnosed right away as soon as you spot that light or warning on your dashboard or gauge cluster.
The sooner you get the check emission system light issue fixed, the better off you’ll be in preventing further damage to any of the components or systems involved.
Frequently Asked Questions (FAQs)
