What Does the Check Engine Light Mean?

The Check Engine Light (CEL Light) is a critical part of your vehicle’s onboard diagnostic system that alerts you when one or more of your car’s systems needs repair or immediate attention.

How does it work?

Modern vehicles are equipped with a series of sensors that monitor the functioning of different components and report the data back to the Engine Control Unit (ECU). When the sensors report any irregularity or malfunction, the ECU activates the Check Engine Light on your dashboard.

However, your car’s Check Engine Light is a general malfunction indicator light — it doesn’t directly tell what’s wrong but signals that your vehicle needs attention.

This malfunction indicator light may also appear differently on different car makes and models. It could be a yellow, orange, or red colored outline of a car’s engine and sometimes have the text “check engine” next to it.

6 Reasons Why The Check Engine Light May Come On

Here are the six common reasons that can trigger the check engine light of your vehicle: 

1. Loose Gas Cap

Sometimes, an issue as simple as a loose gas cap can cause the onboard diagnostic system to turn on the engine light. 

Your gas cap is part of a sealed evaporative emissions system that prevents the gas vapors from escaping into the air. If you accidentally leave this gas cap loose, you can lose fuel through evaporation, and your exhaust gas recirculation system will not function smoothly.  

2. Failed Oxygen Sensor

An oxygen sensor measures the unburnt oxygen in your vehicle’s exhaust system. This helps the ECU to know how efficiently the fuel burns during combustion. Accordingly, the ECU creates an ideal air-fuel mixture to offer optimum fuel economy under different driving conditions.

However, these exhaust system sensors have to tolerate extremely high temperatures and are prone to fail when your vehicle crosses 80,000 miles. 

A faulty oxygen sensor can even damage your spark plugs and catalytic converter, leading to costly repair. Your vehicle will also not pass the emission tests as a bad catalytic converter will release harmful compounds like hydrocarbons and nitrogen oxides. 

3. Weak Car Battery

When your car battery is weak or isn’t fully charged, it’ll fail to send enough power to your vehicle’s ECU. This will result in an illuminated check engine light, and your ECU will register a diagnostic trouble code (DTC).  

You may also notice an illuminated battery light on your dashboard, indicating an issue with your battery or alternator. When that happens, you should call your service center and schedule a battery service. 

4. Faulty Spark Plugs

A spark plug generates an electric spark to crank your engine. When the spark plugs are worn out, they can lead to ignition problems, reduced fuel economy, and even damage other essential elements of your vehicle.

Bad spark plug wires can also cause a flashing check engine light. In other cases, it could be the ignition coil that’s to be blamed. 

5. Engine Misfire

Spark plugs help ignite the air and fuel mixture in the combustion chamber. If the timing of the spark is off or if the fuel mixture and its compression isn’t right, your vehicle can misfire. 

Some other reasons that could also cause an engine to misfire and trigger the engine light are: 

• A defective ignition coil
• Worn spark plug and plug wires
• A defective fuel injector

If you keep ignoring the check engine light for engine misfires, they can result in severe problems like a mechanical failure of your car’s engine.

These failures can be pretty heavy on the pocket if your mechanical breakdown insurance doesn’t cover it. 

6. Faulty Mass Airflow Sensor 

The mass airflow sensor in your vehicle measures how much air enters the engine. The ECU uses this data to determine the amount of fuel to inject into the combustion chamber. 

If the sensor goes bad or there are any leaks in the air intake tract, you’ll notice an illuminated check engine light. 

The mass airflow sensor is also sensitive to dirt, water, and oil. So, contamination in the air tract can trigger the check engine signal too.

What to Do When the Check Engine Light Comes On

When your car’s check engine light activates, it will either blink or be constantly lit, depending on the severity of the problem.

A blinking (or red light in some cases) light means a misfire that needs immediate attention. Ignoring it could severely damage the engine and catalytic converter. In this case, you should reduce your speed and engine load, pull over safely as soon as you can, turn off your engine, and get your car towed to an auto service center.

But if the light is steady and yellow in color, and there is no performance loss, the problem is not severe. But you should still address it at the earliest. 

Now, before a certified technician performs a car diagnostic test, here are a few things you could try to fix a steady engine light: 

• Sometimes, fixing a loose gas cap will quickly resolve the issue. 
• Examine the vacuum hoses for any visible cracks, leaks, or disconnections.
• Check the battery terminals for secure connections. Clean any corrosion and ensure a proper electrical connection.
• Inspect for fouled spark plugs that could be causing an engine misfire. 
• Lastly, retrieve the diagnostic trouble code stored in the ECU. This can provide more specific information about the issue. For that, you can: 
  • Take your car to a repair shop and pay a diagnostic fee.
  • Go to an auto parts store to get a free diagnostic service.
  • Buy an OBD-II scanner to read the code yourself.

Note: While an auto parts store may offer this diagnostic service for free, it will try to upsell you car parts and repairs that your vehicle may not necessarily need.

4 FAQS on Check Engine Light

1. Is the Check Engine Light and “Service Engine Soon” Light the Same?

No, the check engine warning light and the “Service Engine Soon” light point to different things. 

The Check Engine Light points to potential issues with your engine or related systems, while the “Service Engine Soon” light is a reminder for scheduled maintenance tasks, not indicating a required engine repair.

2. How Much Does a Check Engine Light Diagnosis Cost?

An auto service center or independent repair shop could charge anywhere between $50 to $200 to run a check engine light diagnosis. 

However, this cost doesn’t include the taxes and other fees certified technicians may charge for the engine light diagnosis. The price can also vary depending on the vehicle model and the labor charges.  

Based on an extensive engine light diagnosis, an auto repair mechanic may suggest an engine repair or other related fixes that will cost extra.

3. Is it Safe to Drive With the Check Engine Light On?

If there’s no change in your vehicle’s performance and you see a steady engine light, you can continue driving until you bring your vehicle to a safe location. 

Some modern vehicles may also enter the limp mode, wherein the ECU limits the vehicle’s performance to prevent further damage. In either case, you should drive carefully and your next stop should be an auto repair shop. 

However, if you spot a flashing check engine light, you should pull over immediately. Not doing so could damage essential elements like the costly catalytic converter and possibly create dangerous driving conditions for you.

4. How Do I Reset The Check Engine Light? 

Resolving the underlying issue will clear the diagnostic code (fault code) and reset the engine warning light. You can contact your dealership’s vehicle service department or an auto repair shop to help fix the issue.

If you plan to do it yourself, here are some methods: 

• Disconnect and reconnect the battery
• Get an OBD-II scan tool to reset the code

Just remember that the light and fault code will come back on if the underlying issue isn’t eliminated. 

Pro Tip: Getting a multi-point inspection that includes brake service, alignment service, or oil change service can help prevent future problems.