Top 5 Reasons for Check Engine Light

What is your car trying to tell you when the check engine light turns on? 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, are about to head out on a big road trip or you were just telling a friend how reliable your car is.

Regardless, the check engine light is not something you should ignore. Sometimes, you get lucky and it’s an easy fix. Other times, it signals that a serious repair is needed. Either way, the sooner you can diagnose the issue, the faster you can get your car fixed.

These are some of the most common reasons for check engine lights, according to the CarMD Vehicle Health Index.

Catalytic Converters

Average repair cost: $1,356

Of the top 10 reasons for check engine lights, replacing the catalytic converter comes in at number one. 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.

A failing catalytic converter would be noticeable as it may cause reduced acceleration, sluggish engine performance, dark exhaust smoke and heat under the engine.

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Oxygen Sensors

Average repair cost: $243

Oxygen sensors can fail due to lack of maintenance like neglecting oil changes.

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.

Many drivers ignore the O2 senor because it doesn’t typically affect how the car performs, but if you don’t address the issue, it will reduce your fuel economy and slowly damage you car. It could eventually bust your catalytic converter, a much more expensive repair.

Ignition Coils and Spark Plugs

Average repair cost: $387

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 over time.

Spark plugs can usually last up to 100,000 miles, but they don’t always. A good mechanic will inspect them routinely.

If you ignore spark plugs long enough you could experience reduced engine power and fuel economy or damage the catalytic converter.

Loose or Damaged Gas Cap

Average repair cost: $25

As the price tag indicates, this is an easy and inexpensive fix for your check engine light.

If the gas cap is broken or loose, the vehicle’s fuel system gets thrown out of whack. You probably won’t feel or notice anything different about how the car drives, so it may come as a surprise.

If your check engine light comes on without warning, the first thing you should do is tighten the gas cap and check it for cracks. Rather than bring it to the shop, this is a repair you can do yourself. A new gas cap should cost you less than $15.

Mass Air Flow Sensor

Average repair cost: $319

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.

Like some of the other problems on this list, an unchecked mass air flow sensor will decrease your fuel economy and can damage your spark plugs, catalytic converter and oxygen sensor, resulting in a mega-repair with a big price tag, so don’t hesitate to bring to a trusted mechanic.

Lesson learned? Avoid the dreaded check engine light by staying up on your car maintenance.

If you need a  mechanic, search our list of reputable AAA Approved Auto Repair facilities in your area.

And if your check engine light turning on is the latest in a series of ongoing car problems, it may be time to invest in a new set of wheels. Learn about the benefits of the AAA Auto Buying program.

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22 Thoughts on “Top 5 Reasons for Check Engine Light

  1. Notice that the top 5 reasons all pertain to internal combustion engines. None are applicable to electric vehicles. Please think about purchasing a new or used EV when the time comes to replace. If the added maintenance doesn’t move you to an EV, $5/gal gas should!

    1. Gross, I’d rather pay thousands more per year on car maintenance than drive anything other than my loud gas guzzler.

  2. Surprised neither article nor comments (maybe one did..?) mentioned what has to be really common – and this is according to my mechanic, who I trust – at least with my car, a Corolla, it comes on automatically every 5,000 miles, and stays on til it’s serviced in some way. That could be ‘overdue for oil change’ on poster mentioned. It’s been on for a couple months now in fact and likely will stay that way as I tend to change the oil once a year, not twice. Maybe it should do it more often?

  3. Just a warning/comment and nothing to do with the check engine light …

    If you happen to go to a dealership for any repairs keep a watch as to when the car is actually taken in and bought out after the repair (not an oil change). They would normally park it in their lot once done – wait for a while, sometimes a couple of hours before they call you.
    It has happened to me at a reputed dealership – I was charged labor for 3 hours when the car was physically inside for only an hour.
    When I showed them proof they gave me a refund….so please keep a watch on your car (if for repairs), do not go and sit in their lounge drinking their coffee and having their bagels … that dollar worth of freebie is costing you hundreds.

  4. The reason for the Check Engine Light could be secondary.
    Twice (Dec 2017 and May 2022) that light came on and the reason was that squirrels or chipmunks had chewed through some wires in the engine compartment of my Audi. Be careful with cars parked outside if “rodents” are around. NOTE No other cars that we have had, including my wife’s Honda Fit (which are parked near the Audi, outside) were ever affected.

  5. My check engine light came on-(car 19 years old). Gas station changed the gas cap, and gas cabinet (from topping off the gas tank which is said isn’t good to do. When it came on again; he did some underneath repairs; hose. Total around $800. I now purchased a new car. I had low mileage.

  6. 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.

  7. 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.

  8. 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

  9. 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?

  10. 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?

  11. 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 ?

  12. 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?

    1. 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!
      -Dana

  13. 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.

  14. 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.

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