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Top 5 Reasons for Check Engine Lights Coming on

What does a check engine light mean for your vehicle? Find out the most common reasons and how to identify the source.

reasons for check engine lights

The check engine light always seems to flash at the worst possible time. Either you just got your car out of the shop or you were just telling a friend that your car is super-reliable.

Regardless, the check engine light is not something you want to ignore. Sometimes, you get lucky and it’s an easy fix. Other times, it signals that a serious repair is needed.

Keep reading to find out the most common reasons for check engine lights coming on in cars, according to the 2017 CarMD Vehicle Health Index. If you’re reading this, chances are it has happened to you (maybe even today!). But don’t worry. By the time you’re done, you’ll have more than a handful of possibilities to check or have your mechanic check. You’ll be well on your way to fixing your car!

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5 reasons for check engine lights

Oxygen sensors

Average repair cost: $258.63

Of all the cars currently on the road, model year 2005 vehicles are the most likely to have check engine lights come on, according to the CarMD study. The median age of cars in need of check engine light repairs is 11.9 years old. And not to scare you, but these repairs can get pricey. According to the study, the average repair related to a check engine light is $397.87.

Of the top 25 reasons for check engine lights coming on, faulty oxygen sensors are at the very top of the list. In fact, they account for 8 percent of all check engine lights in cars.

Most cars have two to four oxygen sensors. They measure the amount of unburned oxygen in the vehicle’s exhaust system. If one fails, the vehicle could burn more fuel than is needed. This will result in a decrease in fuel economy of up to 40 percent, according to the U.S. Department of Energy, and that means more money out of your pocket!

This repair could cost you over $250, but if you don’t address the issue, it could bust your catalytic converter, a much more expensive repair that we will cover next.

Catalytic converters

Average repair cost: $1,190.18

The catalytic converter turns carbon monoxide found in exhaust gases into less harmful emissions. Without it, your car won’t go, and if it does, it will get horrible gas mileage. The good news is this part should not fail if you are keeping up with your car maintenance. The main reasons for it failing are faulty oxygen sensors and bad spark plugs, which your mechanic should be checking when your car goes into the shop. This is one of the reasons AAA stresses preventative maintenance and creating a rainy day fund for car repairs to its members. A recent AAA study found that 1 in 3 Americans would not be able to afford a car repair without going into debt.

Ignition coils and spark plugs

Average repair cost: 401.22

These parts help start a car. The spark plugs ignite the mixture of air and gas found in the combustion chamber and spark plug wires deliver the spark needed to do this from the ignition coil to the spark plugs. All very cool stuff, and all parts that degrade and could fail in a car over time. In vehicles built this century, spark plugs can last up to 100,000 miles, but they don’t always. A good mechanic will inspect them routinely. Having bits and pieces of this system replaced should cost you less than $400. But ignore spark plugs long enough and you could damage or ruin the catalytic converter. Other symptoms of failing spark plugs or ignition coils include reduced engine power and fuel economy.

Loose or damaged gas cap

Average repair cost: $16.88

As the price tag indicates, if this applies to you, thank your lucky stars. Among the top reasons for check engine lights coming on, this is as good and cheap of an outcome as any (other than the light turning off, which you should probably still get checked out to be safe!). Basically, if the gas cap is broken or loose, the vehicle’s fuel system gets thrown out of whack. The difference between this and the other reasons for check engine lights coming on is that you probably won’t feel or notice anything different about how the car drives. If you suspect this is the case, pull over, tighten the cap and look for cracks. Rather than bring it to the shop, this is a repair you can do yourself. A new gas cap at the auto parts store should cost you less than $5.

Mass airflow sensor

Average repair cost: $378.15

This sensor works with the vehicle’s computer to get the right mix of air and fuel in the combustion chamber. If it fails, so can your car. Rough idles and stalls are all but assured when this happens. But like some of the other problems on this list, this reasons for check engine lights can also damage other parts and lead to a mega-repair with a big price tag. A failed mass airflow sensor can damage spark plugs, the catalytic converter and oxygen sensors. It will also decrease your fuel economy. The labor is a bit easier for this one compared to some of the others on the list, but bring it to a professional if you are unsure about messing with your car.

The check engine light is one of the biggest nuisances for drivers. Stay up on your car maintenance and you should be able to avoid most of the headaches on this list.

If you need a  mechanic, search our list of reputable AAA Approved Auto Repair facilities in your area. If this is just the latest in a series of problems with your vehicle and you have been thinking about a new set of wheels, click here to learn about the AAA Auto Buying program. AAA members save an average of $3,106 off MSRP on new cars and get used car discounts with the program.

And if you have survived one of these terrible car problems, tell us about your experience in the comments section below. 

  • I just had that issue “check engine light on”. Auto mechanic connected scanner and code p018 was in memory indicating low coolant temp. Test drove vehicle and temp only rear 170. Installed new thermostat, added coolant and cleared code. Test drove and temp is now 194. Problem fixed. Total repair cost was $132.39.

  • The manufacturer should list these in the manual. The first time it happened to me, all it said was go to the dealer immediately. Turns out it was the gas cap. I was charged over $100. I was very angry. It happened again some years later right after I left a gas station. My car started running rough. I though it was a major problem at first. I tightened the cap. Turns out that was the problem. Didn’t have to go to the mechanic.


    After having all of the o2 sensors replaced, new coil pack and spark plugs, and everything else mentioned here check engine light is still on after $1,050 in repairs. PCM… came up when checked. I was told it was the computer board. Does this make any sense to you?

    • Hi there!
      I’m going to check with our resident car guru, AAA’s Car Doctor John Paul, and see what he has to say. Stay tuned!


    I think you left out the most common reason of all — overdue for an oil change!

  • Recently I signed up through my auto insurance for a safe driving monitor device. I plugged the device into my auto soon after it arrived. The check engine light came on. When after a few minutes the check engine light didn’t go off, I unplugged the device from the auto. The light continues to stay on. What should I do now – ignore or take it to an auto repair service ?

  • Had a remote starter installed and a year later the check engine light came on. Dealer couldn’t figure it out and said it could be the remote starter . Wanted to charge me over $400 to disconnect the remote. Has anyone experienced this?


    My check engine light came on after the vehicle stalled twice while driving. Brought it to my dealer for service, they hooked it up to diaognostic and said they can’t be sure but to find out they wanted to rip the front end off my car and it will cost $150.00/hr in labor and it will take 5 hours to do it, for not being even sure it was a sensor? Is it normal for a dealer service center to Not Be Sure?


    Most parts chains (like Auto Zone) offer free code reads. I had one on my truck for a misfire, that was a $76 part that took me 5 minutes to install. Often it is the gas cap, usually just tightening it right works. I had to replace one once, it was $12 online. But get the free read to see if you can do it, or even just what it is


    PCM is the computer. All those sensors have to go somewhere. How old is your car? Emissions parts (which some of these are) are covered longer than your standard bumper to bumper coverage. 5/50 for all cars and makes, it is the law.

  • My mechanic said it is the catalytic converter. Thanks for all this info. Now worth a conversation with my mechanic. Maybe it is not the converter, and hopefully one of the other less expensive problems.


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