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

The Check Engine Light (CEL) is never good news about your car. At a minimum, something as simple as your gas cap may be loose, to a failed sensor, wiring, connector, or worse yet, a larger engine problem. Ignoring it if the car runs and drives fine may seem reasonable for some time, but problems will continue to mount. 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 is doing one of two things – it’s either sending a signal to the vehicle’s ECU that something is wrong, or the vehicle’s ECU (computer) itself has lost an accurate read on that particular sensor – meaning, the sensor has gone out. Sometimes, multiple sensors send signals, indicating clues that something bigger has gone awry. You can still drive with the light illuminated, assuming the car is running and driving fine, 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

What’s the Difference Between Checking Yourself and Taking it to a Shop?

An automotive professional shop invests thousands of dollars in diagnostic equipment and training for said equipment. While yes, both yourself and AutoZone can read the engine computer’s stored codes, and through Fix Finder, determine likely causes based on thousands of mechanics intel, none of these can pinpoint with 100% accuracy exactly why the CEL came on.

Many times, multiple sensors are telling us something, but not telling us why, and in nearly all these cases, the vehicle needs to go to a mechanic, where more specialized diagnostic equipment can diagnose the vehicle while the car is running, and determine exactly what is going on beyond just trouble codes. A code reader and an ASE-certified Master Tech with thousands of dollars in diagnostic equipment are NOT the same equivalent at all.

If there is any speculation or doubt about exactly what’s going on with the trouble codes retrieved, you can search here for AutoZone preferred repair shops in your area that can help take the information you already have, and assist you in the repair. It’s important to note that some trouble codes can and do lead to an easy result and solution. Many cannot, and even the most avid of DIYers need the assistance of professional diagnostics.

5 Reasons Your Check Engine Light May be On


Failed Oxygen (O2) Sensors

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

Oxygen sensors also live a hard life in +800 degree exhaust temps and are prone to fail once over 80,000 miles. It’s important to note here that there are a multitude of engine codes where the oxygen sensor is telling us something is wrong, but that DOES NOT mean that the oxygen sensor is bad. Be careful determining that a sensor is truly at fault, as opposed to determining something else is happening. This cannot be more apparent than in the ever-popular PO420 / PO421 engine code, which is detailed HERE.

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 – EVAP

Your gas cap is part of an entire, sealed system that recirculates gasoline vapors from your gas tank and keeps them from escaping into the air. Because of this, this system of lines and valves is closely monitored, and one of the key sealing pieces of this is the gas cap. If you leave your gas cap off accidentally, you’ll notice this light rather quickly. These leaks in the gas tank recirculation (also known as EVAP emission control) system are often grouped as “small” or “large” leaks. Its important to note that the problem is not always the gas cap, but could be one of the plastic recirculation lines, fittings, or connectors causing the leak, but many times, the gas cap is the culprit.

What happens if I don’t replace it?

  • You can lose fuel through evaporation
  • Your recirculation system will not function properly
  • You will have to stare at the CEL until it’s fixed

Bad Catalytic Converter

Your catalytic converter helps protect our environment by converting harmful carbon monoxide into carbon dioxide. Your downstream oxygen sensors carefully monitor the converter’s performance, and if something goes awry, a number of trouble codes based on “Catalyst efficiency” will appear. It’s important to note that a catalytic converter will more than likely not fail on its own. Generally, something causes it to fail, or not perform properly. Because of this, simply replacing the converter or the oxygen sensors around it will often not fix the problem, and often leads to yet another failed converter. Things as serious as a blown head gasket can force burned coolant vapor into the exhaust, triggering a catalyst engine code.

What happens if I don’t replace it?

  • Your vehicle will not pass an emissions test
  • You will experience reduced performance and fuel economy
  • Whatever caused the converter to fail initially will continue to get worse, possibly causing engine failure

Mass Airflow Sensor or Intake Tract

Your mass airflow sensor (MAF) measures the amount of air entering the engine to determine how much fuel is needed to run your engine properly. If there are any leaks before or after the mass airflow sensor in the intake tract, a light can trigger. Mass airflow sensors are sensitive to oil, dirt, and water vapor, and any contamination on them can trigger a light. Sometimes, cleaning the sensor with intake / mass airflow sensor cleaner can fix the issue. Other times, the sensor needs to simply be replaced. Be sure to check the intake ducting carefully for leaks, rips or tears/damage.

What happens if I don’t replace it?

  • A faulty MAF sensor can cause rich or lean conditions, which can lead to further failures of other parts
  • You will experience reduced performance and fuel economy


Your spark plugs ignite the air/fuel mixture in the combustion chamber of your vehicle, but if the timing of that spark, the mixture of the fuel and compression of the air/fuel isn’t perfect, you will get a misfire. Multiple issues can cause a misfire either on one particular cylinder, or a “multiple cylinder misfire” meaning, the misfire is random.

  • Defective ignition coil: (either DIS or coil-on-plug). A quick trick to determine if a coil is causing a misfire is to simply move the coil and swap with another on the engine. If you have a #2 misfire and move the #2 coil to #4, for example, then the misfire moves to #4, you know the coil is suspect.
  • Defective fuel injector: If a fuel injector fails to deliver the proper amount of fuel to a cylinder, you will have a misfire. Unlike swapping the ignition coil with a neighboring coil, a fuel injector is more difficult to move.
  • Vacuum leak: Vacuum leaks often cause multiple cylinder misfires, or lean running conditions. They also can be very difficult to diagnose.
  • Worn spark plugs and/or wires: Sometimes, spark plugs can go far past their life cycle, and fail, causing a misfire. On vehicles with spark plug wires, a defective wire or wires can also cause this.
  • Bad compression: More serious, a burnt or bent valve, worn piston rings, or valve timing that is off will cause a misfire as well. These issues will all become apparent during an engine compression test.

What happens if I don’t replace it?

  • You will experience poor performance (reduced power, engine missing) and reduced fuel economy
  • The misfire will only get worse, and will lead to mechanical failure in the engine

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. As stated before, if you need help from a professional to further diagnose an issue, check out our list of preferred shops in your area.

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

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