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Check Engine Light On: What It Means And How To Fix It

The check engine light is part of your car’s so-called onboard diagnostics system.

The check engine light can mean many different things, from a loose gas cap to a seriously misfiring engine.
Credit: commons.wikimedia.org

Nearly every driver has seen it once in their driving career. But every time you see it, it likely strikes you with fears that a major repair bill is on the horizon. While the check engine light can be intimidating, you can learn more about what causes the light to illuminate to prepare yourself for these circumstances better. With this article, you’ll also learn about how to “fix” your check engine light, depending on what problem caused the light to appear.


The check engine light (properly known as the malfunction indicator light or “MIL” for short) acts as a vehicle’s primary method for communicating an internal problem. Perhaps the most misunderstood light on the dashboard, the check engine light doesn’t actually indicate a specific problem with your car (as the low fuel or low oil lights do). Instead, the check engine light illuminates if your vehicle’s on-board diagnostic system (OBD-II) senses a malfunction in any of the several systems it tracks. This, in turn, produces an error code, which is stored in the vehicle’s main computer until you are able to retrieve it and decode it.

In the past, the check engine light indicated that a problem had arisen without any further details. Today, home mechanics and specialists alike have access to OBD-II scanners that provide actionable codes straight from your vehicle’s on-board computer without needing to take the care apart. With this type of scanner in hand, you’ll be able to diagnose your check engine light with precision.


Many newer vehicles use a pictogram of the engine as the check engine light.
The check engine light is easy to identify and appears in one form or another on all modern vehicles.
Credit: www.flickr.com

Because of its integration with the on-board diagnostic system, the check engine light may indicate a broad range of problems for your vehicle. These can range from simple issues, like a loose gas cap, to a more serious problem, like a misfiring engine. The more you know about the check engine light, the more you’ll be able to diagnose as soon as the light illuminates. More causes of check engine light illumination are listed below.

The check engine light is easy to identify and appears in one form or another on all modern vehicles. Many newer vehicles use a pictogram of the engine as the check engine light, while older vehicles tend to use verbal prompts such as “Check” or “Check Engine.” In either case, the check engine light will always be illuminated with a gold, orange, or red light, alongside other warning lights on your dashboard.

In order to know precisely what your illuminated check engine light indicates, you will need to use an OBD-II scanner to acquire the pertinent error code or seek out a consultation with a trained mechanic. However, before you seek either method of diagnosis, you should note the color and pattern of flashes that your check engine light is presenting.

Check engine light is steady

If the check engine light is steady (or in other words, not flashing), the problem that triggered the light is not immediately serious in nature. As such, you can assume that it is safe to continue driving to your destination. When you arrive, though, you should make an appointment with your mechanic before the problem becomes worse.

The check engine light is flashing

If the check engine light is flashing (or, in some older vehicles, colored red), you very likely have a serious automotive problem on your hands. Generally, a flashing check engine light communicates that your engine is misfiring, which causes unburned fuel to leak into the catalytic converter. This, in turn, raises the catalytic converter’s temperature exponentially, causing permanent damage and expensive repair if not resolved swiftly.


Each car problem that precipitates the check engine light to illuminate is different, necessitating different methods for fixing the underlying problem. Some problems can be fixed in mere minutes, while others may take time or professional support to resolve. In either case, be aware that the check engine light may not shut off immediately. Once several trips after fixing the problem have passed, expect the light to turn off.

Loose gas cap

A loose gas cap is the most common cause for the check engine light to illuminate.
Credit: commons.wikimedia.org

A loose gas cap is the most common cause for the check engine light to illuminate. This can be fixed by simply tightening down the gas cap while the vehicle is parked. If the problem persists, you may have a faulty gas cap caused by the deterioration of the internal O ring. In this case, you will likely need to replace the cap. If a simple tightening fixes the problem, though, no further attention is warranted.

Spark plugs and wires

Another low-level issue that may trigger the check engine light is a problem with your spark plugs and their associated wiring. Spark plugs ignite the air/fuel mixture in your engine, so a problem with them can cause the engine to act out of the ordinary. Spark plug problems may cause the engine to misfire, decreasing fuel efficiency overall.

With some training, these spark plugs and coils can be replaced at home. However, mechanics are just as able to perform the task quickly, as spark plugs are expected to wear down over your vehicle’s lifespan.


Oxygen (O2) sensor

Part of your vehicle’s larger anti-pollution system, the O2 sensor measures how much unspent fuel is escaping into your vehicle’s exhaust fumes. In turn, the sensor sends data to the on-board computer to better regulate fuel usage. A faulty O2 sensor may send incorrect data, resulting in decreased fuel efficiency and increased emissions. In some states, it may cause you to fail a required emissions test.

O2 sensors tend to run for $175 per item and can be replaced by a mechanic. Keep in mind that many modern vehicles include multiple O2 sensors.

Mass airflow sensor

Without mass airflow, your vehicle may have trouble adjusting to changing air conditions.
The mass airflow sensor (MAF) monitors how much air is entering the engine.
Credit: commons.wikimedia.org

The mass airflow sensor (MAF) monitors how much air is entering the engine. Without it, your vehicle may have trouble adjusting to changing air conditions. In addition, a faulty MAF sensor may result in difficulty starting the vehicle, recurrent stalling, and sudden changes in the throttle pedal’s position.

Replacement of this sensor will put you out $120 or more and can be accomplished by a mechanic or other trained specialist.

Catalytic converter

One of the most serious problems a check engine light can signal is trouble with your catalytic converter. This device (included in all modern petroleum-burning vehicles) converts poisonous carbon monoxide to carbon dioxide, thus decreasing the environmental pollution.

Regular maintenance – such as driving at highway speeds and replacing your oil – will keep this part in working condition. If a replacement is warranted, you may be put out as much as $600 or more. You are required to have a functioning catalytic converter in all states, so removal without replacement is not an option.

In total, this is not an exhaustive list of every systematic problem that may cause the check engine light to appear. In order to fully diagnose the extent of your vehicle’s problems, you will need to take your vehicle to an automotive shop and consult with a trained specialist. Many mechanics and manufacturer specialists make use of specialized tools that hook up to your vehicle’s main computer and pinpoint each problem.

Read more: How To Check Brake Fluid Of Your Vehicle


 If the light appears unblinking while driving on the interstate, you do not need to pull over immediately and call for a tow truck.
The check engine light means there’s a problem somewhere in your emissions system. 
Credit: commons.wikimedia.org

The light appears unblinking

Mechanics agree that the appearance of the check engine light is not generally an immediate cause for concern. For example, if the light appears unblinking while driving on the interstate, you do not need to pull over immediately and call for a tow truck. If you sense something is amiss with your vehicle’s handling or performance, though, take proper precautions to ensure your safety.

The check engine light appears blinking

On the other hand, if the check engine light appears blinking, you are likely to have an immediate problem on your hands. Pull over when safe to do so and evaluate signs of engine over-exertion, including temperature and unfamiliar smoke.

In either case, you should make a point to schedule an appointment with your mechanic as soon as possible. The longer you drive with the check engine light on, the more likely you will exacerbate the underlying problem.

Ignoring the check engine light altogether may result in permanent damage to key vehicle components. This results in dramatically decreased fuel efficiency, increased environmental pollution, and decreased (or permanently disabled) vehicle functionality overall.


Even if you do not immediately sense a problem with your vehicle, the check engine light should not be taken lightly. In many cases, the check engine light is your first signal that something is wrong with a key vehicle system that should be evaluated as soon as possible. The causes of a check engine light appearance are numerous, but each can be fixed with swift action and (usually) a modest cost. In order to ensure your vehicle’s long-term health, don’t sleep on a check engine light. You’ll be glad to have acted early to resolve the problem, using the knowledge and information you’ve learned about the check engine light and its causes.

Want to learn more about other light on problems and how to fix it? Check out these useful links for more knowledge

Why Is My Airbag Light On?

ABS Light On: Why? What To Do?

Brake Light On: Common Reasons Why and How to Fix Them

What To Do When Oil Light Comes On A Car?

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