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What are the 6 Most Common Reasons the Check Engine Light is On?

Power - by Bryan Veldboom - updated on 12/30/2021

Vehicle dashboard with a check engine light on

We've all felt that sinking feeling that happens when your vehicle's Check Engine Light (CEL) suddenly lights up without warning. Sometimes when this happens it's obvious from the vehicle's performance that something is wrong. Other times, the CEL will light up even when everything seems to be working fine. So, how do you determine why the CEL has turned on? Today, we'll be discussing six of the most common reasons your check engine light may be on, along with additional symptoms to help you identify the problem. We'll also be taking a look at the ZUS Smart Vehicle Health Monitor and how you can use it to easily read and identify vehicle error codes on your own.

Common Check Engine Light Causes

When your CEL lights up, it can be caused by a wide range of different issues, from simple fixes to more serious repairs. Here are six of the most common causes.

1. Loose or Damaged Gas Cap

Your gas cap performs a number of useful functions. For starters, it helps to maintain the correct pressure in your gas tank. It's also a part of your vehicle's evaporative emissions control (EVAP) system. The EVAP system prevents fuel vapors from getting into the atmosphere.

What Causes a Gas Cap to Fail?

Your vehicle's onboard computer monitors the EVAP system for leaks. If your gas cap is loose, missing or damaged, it will detect the leaking fumes and turn on your CEL.

Troubleshooting Tips

If your CEL comes on right after you've filled up your tank, the gas cap is probably just loose. Tighten it up and see if that solves the problem. Additionally, if you notice that your gas cap is damaged in some way, be sure to have it replaced.

2. Your Oil is Too Low or Too Dirty

Oil lubricates the components of your engine. This reduces the friction and wear between these components, allowing them to run smoothly and helping to protect them from becoming damaged. It also helps to minimize heat between the moving parts and works to keep engine components clean.

Can Low Oil Cause the Check Engine Light to Come On?

Low oil pressure can cause your CEL to come on. This can be caused by not having enough oil or because the oil is too dirty.

Troubleshooting Tips

Check the oil level in your vehicle. If it's too low, add additional oil. If the oil appears dark black in color, have the oil changed as soon as possible.

3. Failed Oxygen Sensor

The oxygen sensor (also known as a lambda sensor) is located in your vehicle's exhaust system. It measures the amount of oxygen present in the exhaust gases that leave your engine. This data is then sent to your vehicle's onboard computer in order to determine the proper air-to-fuel ratio for your engine. An oxygen sensor will alert you when your vehicle's emissions are too excessive.

What Causes an Oxygen Sensor to Fail?

Oxygen sensor failure can occur when contaminants such as silicates and phosphorus enter the exhaust. Without a functioning oxygen sensor, your engine's fuel ratio can be thrown off. Using low-quality fuel that's not recommended for your vehicle can also contribute to oxygen sensor failure.

Additional Signs That Your Oxygen Sensor is Failing

If you have a faulty oxygen sensor you may notice that your gas mileage gets worse over time. This is because your engine's fuel ratio is off. You may also notice that your engine sounds rough or irregular.

4. Failing Catalytic Converter

The catalytic converter cleans your car's exhaust emissions by converting harmful compounds into safe gases.

What Causes a Catalytic Converter to Fail?

Your catalytic converter can become contaminated or clogged by engine coolant and engine oil. A catalytic converter can also overheat due to an excessive amount of unburned gas. A failed oxygen sensor can also cause a catalytic converter to overheat.

Additional Signs That Your Catalytic Converter is Failing

If your catalytic converter becomes clogged or begins to overheat, you may begin to experience issues with your engine. These can include difficulties starting the engine, sluggish performance or a reduction in acceleration. You may also notice dark exhaust smoke that gives off a sulfur smell.

5. Bad Spark Plugs

Your spark plugs create a spark of electricity that ignites the mixture of air and fuel in your engine. This creates the combustion needed to start your vehicle.

What Causes Spark Plugs to Fail?

Deposits of oil, fuel or carbon can form on spark plugs due to their interaction with the air-fuel mix. Spark plugs can also corrode or begin to misfire over time.

Additional Signs That Your Spark Plugs are Failing

If a spark plug is bad, you may have difficulties starting the engine. The engine may hesitate, misfire or make a rattling or vibrating sound when the engine is idling. You may also experience reduced gas mileage or have a hard time accelerating.

6. Failed Air Flow Sensor

A mass air flow (MAF) sensor is part of the vehicle's electronic fuel injection system and helps calculate the amount of air entering the engine.

What Causes an Air Flow Sensor to Fail?

Mass air flow sensors can become contaminated or damaged. When the air flow sensor is faulty, it won't measure the correct amount of air flow and your engine will have a fuel mixture that is either too high or too low in oxygen.

Additional Signs That Your Air Flow Sensor is Failing

Some common symptoms of a failing air flow sensor include difficulty starting the vehicle, as well as the engine stalling, jerking or hesitating during acceleration. You may also notice black colored exhaust or a rattling/vibrating sound when the engine is idling.

How Can You Read Car Error Codes?

As you can see from the examples above, diagnosing auto issues on your can be pretty imprecise. For instance, if you notice hesitation when accelerating your vehicle, it could be because of a failing catalytic converter, a bad spark plug or a faulty air flow sensor. So how can you identify the problem more efficiently? Unless you're good at working under the hood, The only real way to do that is by reading the error codes directly from your vehicle's onboard computer.

The ZUS Smart Vehicle Health Monitor Mini from Nonda allows you to read and clear vehicle error codes. That means you can identify why your CEL is on without having to bring your vehicle to a repair shop. It also provides you with step-by-step DIY repair instructions, allowing you to take care of simple fixes on your own. Plus, you can use it to log your mileage and keep tabs on your engine's health. Learn more about what the ZUS Smart Vehicle Health Monitor can do for you.

Batteries Plus Has Your Automotive Needs Covered

Browse our selection of auto essentials online or in-store. Batteries Plus also carries top-quality batteries for your car or truck, as well as battery chargers to help you maintain them. Plus, don't forget about our expert car battery installation (available on most makes and models at most locations) and convenient free battery testing. Plus, many of our stores also offer key cutting and key fob replacement. Check with your nearest store for availability.

Looking for more helpful automotive information? Read our blogs entitled "6 Reasons Your Car Battery isn't Working" and "6 Essential Car Maintenance Checks and How Often You Should Perform Them."

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