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Check Engine Light: All You Need To Know

Here's where you can find out what to do when the check engine light comes on
If you want to know what to do when the check engine light comes on, this is the right place

We’ve all seen a check engine light glowing on our dashboard, but what does it mean? Just telling us to check our engine doesn’t give us a lot of information. When the light comes on, it usually doesn’t indicate an emergency. Most of the time, you can continue driving your car normally for a little while with a check engine light, but do not ignore it. There are a few common reasons why the check engine light is on, and there are easy ways to diagnose the issue. Oftentimes, the issue isn’t with the engine at all. Understanding what the dreaded check engine light means is the first step to getting it off your dashboard.

check engine light
When the light comes on, it usually doesn’t indicate an emergency.

Reference: I highly recommend reading our Best OBD2 Scanner Reviews & Comparison 2020 article for an in-depth look at some of our favorite diagnostic tools on the market right now.

What Does Check Engine Light Mean?

How to fix check engine light that’s on in your car

The check engine light tells you that the Onboard Diagnostics (OBD) in your car’s computer has detected a problem. The Engine Control Unit (ECU) is constantly monitoring your car’s components, looking for problems and defects. When the ECU notices something wrong, it tells you via the check engine light. Originally intended to make jobs easier for professional mechanics, there are easy ways to figure out what the check engine light is trying to tell you without going to a shop. With the right equipment, you can even diagnose your car in your garage.

Reference: How to reset your check engine light?

Why Is My Check Engine Light On?

The only way to know for sure why your check engine light is on is to get it scanned with an OBD reading device. The OBD system was initially designed to keep emissions systems working correctly. Incidentally, emissions problems are some of the most common defects found by check engine lights. Let’s take a look at some of the most common issues indicated by a check engine light.

infographic 10 reasons why the check engine light on.
10 reasons check engine light on.
Credit: obdadvisor.com

1. Loose Fuel Cap

Did your check engine light come on right after filling the fuel tank? It’s probably because the gas cap is on incorrectly or not on at all. While this common problem has nothing to do with the engine of the car, it triggers a check engine light because it’s technically a problem with the car’s emissions system. A loose or missing gas cap allows gas fumes to escape into the atmosphere, which hurts your car’s fuel economy. Some newer cars are ditching removable gas caps and integrating them into the gas doors. This makes a loose or missing gas cap impossible unless you leave the fuel door open.

2. Oxygen Sensor

The oxygen sensor in your car does just that – it senses oxygen. Specifically, it senses how much unburned oxygen is flowing through your exhaust system. In turn, the sensor talks to the car’s computer to regulate the air/fuel mixture burning in your engine. A broken oxygen sensor can cause an imbalance in air and fuel, which hurts performance and efficiency.

3. Mass Airflow Sensor

Similar to the oxygen sensor keeping an eye on the exhaust, the mass airflow sensor is on the other end of the engine, measuring how much air is coming through the intake. A faulty MAF sensor has similar effects as a bad oxygen sensor, reduced performance, and fuel economy. This sometimes results in much more severe engine damage that’s not as easy to repair as merely replacing a sensor. If you have a bad mass air flow sensor, there’s usually a noticeable difference in how your engine runs.

4. Catalytic Converter

Catalytic converters have been mandatory equipment on cars in the US since the mid-1970s. The catalytic converter is imperative in reducing toxins in your car’s exhaust. Basically, it turns carbon monoxide into carbon dioxide and water. In addition to the downsides of a broken MAF or oxygen sensor, a defective catalytic converter will make your car fail an emissions test if your state requires one. In order to legally keep your car on the road and keep it environmentally friendly, you must keep a functional catalytic converter in your exhaust system.

What If My Check Engine Light Is Flashing?

When your check engine light is flashing, it’s screaming at you that you need to stop driving and get it serviced immediately. And when I say immediately, I MEAN IT! Pull over on the side of the road or in the parking lot and call a tow truck. It may be expensive to have them tow your car to the nearest auto repair shop, but it could prevent further damages from occurring to the vehicle.

What Does A Flashing Check Engine Light Mean?

Engine Misfire

A blinking check engine light could signify an engine misfire that dumps unburned fuel into the exhaust system. If that happens, the catalytic converter’s temperature could be raised to a dangerous level, causing potential damage. Trust me, you don’t want to have to pay a bill for this type of repair. And the longer you wait to get it fixed, the higher the possibility is for something to go severely wrong and cause significant damage. But that’s not the only reason why your check engine light could start blinking.

Other Reasons

You could have a crack in the manifold gasket, a bad coil, a misfiring cylinder, or other major issues.

What To Do When The Check Engine Light Is On?

Don’t ignore it! 🙂

When the check engine light illuminates on your dashboard, don’t panic, but also don’t assume everything’s fine. Get the car diagnosed as soon as possible before any serious damage can occur.

Get It Checked By A Mechanic

If you intend to have the mystery problem solved at a shop by professionals, then take it to a mechanic you trust for diagnosis. They will scan your car with their professional-grade equipment, pinpoint the issue that turned on the light, and hopefully repair it promptly. If you haven’t had your car in the shop in a while, a check engine light can be a good excuse to get it looked over by your mechanic for any other problems you might not have noticed.

Do It Yourself With A Check Engine Code Reader

If you’re a DIY mechanic who does your own service and maintenance, you have a couple of other options. If you don’t own any tools to communicate with the Onboard Diagnostics in your vehicle, you can take it to any parts store and just ask for a scan. Parts stores will gladly do this for you for free in hopes that you’ll buy parts from them to fix the issue. They’ll plug in a scanner – which they probably sell at their store – to the diagnostic port in your car. It will find a code, the associate helping you will look up what the code means, and then you’ll have your answer. From there, it’s up to you to decide if it’s something you can fix yourself or if you’d rather have a professional take care of it.

Are you a garage tech pro and already have a scanner in your toolbox? If so, then it’s time to use it! There are countless different OBD scanners on the market that anyone can buy. Some simply tell you the code; others have full-color screens with detailed information about your car. You can even get software and equipment for your computer to diagnose your car if you need the most data you can get out of your vehicle. Whatever diagnostic hardware you choose, it will come with some way to plug into your car’s diagnostic port. This port is a d-shaped female plug usually located underneath the steering column. The standard system that we use today is called OBD-II. OBD-II codes consist of a letter followed by four numbers. Since OBD-II picks up on far more than just emissions problems, different letters tell you the category of the issue turning on the check engine light.

  • P means powertrain,
  • B means body,
  • C means chassis,
  • and U means there’s a network problem.

These letters are handy because it gives you an idea of what’s wrong before you even look up the whole code. The four-digit number that comes after the letter gives you the specific problem with the vehicle. If the scanner you’re using doesn’t decode it for you, the easiest way to find out what a trouble code means is by Googling it.

The Bottom Line

Do not fear the check engine light! It’s nothing to be afraid of as long as you diagnose and fix the problem it’s trying to tell you about as soon as you can. The last few decades have given us considerable advancements in car technology. One of the most significant is Onboard Diagnostics and the Engine Control Units (ECU) that allow our cars to communicate with us. Keeping our emissions systems in proper working order is one of the surest ways to maintain our cars’ performance and efficiency. OBD makes that maintenance more effortless than ever. An OBD scanner can be one of the handiest tools in your toolbox. Just be careful not to lose it when all of your friends ask to borrow it from you!


Can the check engine light turn itself off?

No. You have to find the CEL codes, fix the issues associated with those codes, and then reset them. The CEL will turn off after that.

Can you drive a car with the check engine light on?

Yes, you can. But it is not recommended because you will be driving a car that obviously has a fault somewhere in the engine system. You should know that if the issue causing the CEL is severe, the car may fail to start or move.

Can a car pass inspection with the check engine light on?

No, unless it had not been subjected to a drive cycle after codes were cleared and issues fixed. In which case, you may have to do a test drive to turn off the CEL and then retake the inspection test.

Tim MillerFounderOBD Advisor

I’m Tim Miller from Denver, Colorado. I’m the founder of obdadvisor.com, an automotive blog about "Auto Diagnostic Tools and Repair Guides". My fan page is facebook.com/autozikcom. I've been working as an automotive mechanic and blogger for over 10 years writing articles to share my experiences and expertise.

Web: https://www.obdadvisor.comEmail: [email protected]
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