Learn the Top 5 Reasons Your Check Engine Light May Be On

The Check Engine Light (CEL) is never good news. At the very least it means you’ll need to service the engine soon, or check some of the auxiliary systems. You may want to ignore it, but the light means there’s a problem that will only get worse the longer you leave it unchecked. Here at AutoZone, we’re pretty familiar with the light, so we’ve rounded up the 5 most common causes and everything else you need to know when you’re wondering, “Why is my Check Engine Light on?”

Is the CEL Lit or Flashing?

If the light is steadily lit, that means that one of your engine sensors picked up suspicious reading. This can indicate a problem with an engine component, or with one of the sensors. You can still drive with the light illuminated, but you should get it checked out as soon as possible. If the light flashes, that means you need to pull over immediately as your vehicle has a serious issue that could cause imminent damage.

Can I Check My Own Light?

You can. All that you need to check your own light is an OBD-II reader, which can pull codes that help you find out what’s wrong with your car. If you don’t have a code reader, keep reading to learn the most common causes, and then bring your car to your local AutoZone to find out why your Check Engine Light is on.

Check Your Own CEL

5 Reasons Your Check Engine Light May be On


Your oxygen (O2) sensor needs to be replaced

Your oxygen sensor measures the amount of unburned oxygen in your vehicle’s exhaust system.

What happens if I don’t replace it?

  • Your engine will burn more fuel than needed
  • You will lose fuel economy, and get fewer miles per gallon
  • Faulty sensor(s) can cause damage to your spark plugs and catalytic converter

Gas cap is loose, damaged, or missing

Your gas cap seals the fuel system and helps maintain pressure within the fuel tank. It also prevents gasoline fumes (hydrocarbons) from being released into the atmosphere while you aren’t driving your car.

What happens if I don’t replace it?

  • You can lose fuel through evaporation
  • You may take more trips to the gas pump

You have a bad catalytic converter

Your catalytic converter helps protect our environment by converting harmful carbon monoxide into carbon dioxide. Damage usually occurs due to neglected maintenance (like the parts in reasons # 1, 4, and 5).

What happens if I don’t replace it?

  • Your vehicle will not pass an emissions test
  • You will experience reduced performance and fuel economy
  • Your car may run at a higher temperature

It’s time to replace your Mass Airflow Sensor (MAF)

Your mass airflow sensor measures the amount of air entering the engine to determine how much fuel is needed to run your engine efficiently.

What happens if I don’t replace it?

  • Faulty sensors can cause damage to spark plugs, O2 sensors or catalytic converter
  • You will experience reduced performance and fuel economy

Your vehicle needs new Spark Plugs or Plug Wires

Your spark plugs ignite the air/fuel mixture in the combustion chamber of your vehicle. The spark plug wires deliver the spark from the ignition coil to the spark plugs.

What happens if I don’t replace it?

  • You will experience poor performance (reduced power, engine missing) and reduced fuel economy
  • Worn plugs and plug wires can cause clogged catalytic converter or damage to ignition coils and O2 sensors

Does AutoZone Scan Check Engine Lights?

AutoZone does. If your light is on, and you are wondering why, head down to your local AutoZone where one of our store associates can help diagnose the issue through our free Fix Finder service.

Regardless of whether the light is glowing or flashing, you can find out what’s causing the issue by bringing it in for a Fix Finder reading.

These are the most common reasons, but there are others too. Finding out why your Check Engine Light is on may help you save gas and avoid more problems down the road. Get in the Zone.

The AutoZone Fix Finder

Fix Finder is an easy-to-use tool to troubleshoot why your Check Engine Light is on using an extensive database of repair solutions from ASE certified mechanics to help you find the most likely fix.

How does AutoZone Fix Finder work?

  • Fix Finder reads diagnostic trouble codes from your vehicle’s onboard diagnostic system, and pulls your vehicle information and mileage.
  • This data is analyzed to give you vehicle-specific fix solutions that have been verified from the records of over 5.5 million ASE-Certified technicians.
  • AutoZone gives you a FREE, easy-to-read MYZONE HEALTH REPORT, which includes relevant codes, and details the action that needs to be taken.

Let us help you find out if the problem is simple or serious

Your Check Engine Light may be indicating a simple problem that’s easy to fix—such as a loose gas cap that needs to be replaced. It could also be a warning of a serious problem that could damage your engine components.

Your Check Engine Light blinks or glows depending on what’s wrong with your vehicle. A flashing Check Engine Light indicates that your vehicle’s engine is in serious trouble, whereas a steadily lit one means less urgent service is needed. Regardless, it’s easy to find out what the issue is, and taking care of it quickly can save you money, and prevent further damage down to the road.

Common Questions

What is the Check Engine Light?

This light is the centerpiece of your vehicle’s on-board diagnostics system (OBD). Since the 1980s, many vehicles have been built to be run by the engine computer unit (ECU), which controls vehicle’s drivetrain and auxiliary systems based on input from the vehicle’s sensors. This computer will make adjustments to your vehicle’s performance to keep your ride running smoothly. When it runs into an issue that the ECU cannot correct, the check engine light (CEL) starts glowing. The light is typically yellow or orange, and features an image of engine with text telling the driver to check or service the engine or powertrain. The engine computer stores a diagnostic code when the light begins glowing.

The light can be intimidating and most people prefer to just not think about it, but not thinking about it is the worst thing you can do. That’s because the light indicates that one part or system in your vehicle is not functioning correctly. And driving a vehicle that is only partly functional is a can cause a lot of damage over time.

Part of what makes the light so intimidating is that it is often misunderstood and difficult to understand. For starters, the light itself is incredibly vague as it can indicate a wide variety of issues. Luckily the system logs diagnostic trouble codes (DTC), but these sometimes only provide a starting-off point for further diagnosis. That’s because even though the code indicates that a certain issue has arisen, it may mean that a part is broken, a sensor is broken, the sensor is connected to a broken wire, the wire is connected to a broken relay, etc. For example, The oxygen sensor may detect a lean air-fuel mixture. This could indicate that your vehicle’s fuel system is not providing enough fuel for proper combustion, or it could be caused by a faulty sensor. These sorts of diagnostics only become more complicated as you look at parts that interact with more systems.

What Does a Lit Check Engine Light Mean?

It’s a question we’ve all asked at some point. The engine service light starts glowing and the driver is left confused, scratching their head to try and make sense of what’s wrong with the vehicle. At it’s most basic, a lit CEL means that one of your vehicle’s sensors has detected a problem with the vehicle’s engine or powertrain, and stored a trouble code detailing the issue. An OBD-II tool can read diagnostic trouble codes that communicate what is wrong with your car.

We use one of these in our Fix Finder service, where we will scan your car when the check engine light is lit to find out what part you need to fix your car. With stores in all 50 states, we’ve scanned a lot of vehicles. We’re more than a little familiar with the types of issues that cause the check engine light to become illuminated, so we’ve compiled a list of the top 5 reasons why the check engine light is on.

OBD-II readers and services like AutoZone’s Fix Finder are pretty useful for diagnosing the cause of a glowing CEL. That’s because there are many potential causes that vary in severity. Most cars will illuminate the light if the gas cap is loose, but they will use the same light to indicate an engine that is misfiring. A loose gas cap is not that big of a deal: the best case scenario sees you tightening the cap, while the worst case is you need to replace it. Engine misfiring, on the other hand, can cause seriously damage your engine over time.

What Does a Flashing Check Engine Light Mean?

While the light appears the same way regardless of the part or system affected, it does differ depending on severity. If the check engine light is illuminated, you can keep driving but you should get the car checked out as soon as you get a chance. A flashing check engine light means that you need to pull over immediately as your vehicle has a serious issue that could cause severe engine damage.

Generally, a flashing CEL indicates that the engine is misfiring, which can damage your engine or catalytic converter. Both of those can be pricey and complicated to repair, hence why it’s best to pull over immediately. The misfires are most commonly caused by defects in the ignition and fuel systems, or inside of the engine.

How to Reset the Check Engine Light


Drive Till it Turns Off

This is the easiest way. Your vehicle’s sensors will generally recheck the problem that led to the CEL. If you’ve fixed the issue, the light will go off once your ECU recognizes this. If it stays on for more than 3 days, you may need to double check your fix or reset the light manually.


Turn the Car Off and On 3 Times

Some vehicles reset the trouble codes when they are turned on and off three times in a row. To do this, simply turn the car on for a second, and then off for a second. Repeat two more times and then check to see if the light reset.


Disconnect and Reconnect the Battery

Pop the hood, disconnect the positive power cable from the battery, and wait 15 minutes. Then put your key in the ignition and turn it to the on position 3 times to clear the data. Lastly, reconnect the positive cable and start the car. The light should clear in about a minute.


Use an OBD Reader

Some code readers can clear the codes. If you have one that is capable, plug it in, select any error codes and use the option to erase the code.

No matter how you choose to reset the code, make sure to fix the problem first. If you don’t, the light will come back on as soon as the ECU detects the issue again.

More Diagnostics

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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