The five most common reasons your Check Engine light turns on

Check engine light symbol

Check engine light symbol

The Check Engine light is a universal point of frustration for all car owners. When the light turns on, there’s no indication as to what the problem is or where it lies. It’s just an annoying bright orange engine symbol that taunts you until you give in and take your car to the shop.

Thankfully, we have some (mostly) good news. More often than not you can either fix the Check Engine light problem yourself or take your to the shop for a relatively inexpensive repair. However, if you choose to ignore the light completely and keep running your car as usual, repairs that could have been fast and inexpensive can quickly turn into stalling and expensive shop work.

Here are the top 5 reasons your Check Engine light appears and what you can do in response (other than panicking).

Car oxygen sensor

Car oxygen sensor

A faulty oxygen sensor

If you’re one of the 9 million Americans who let their Check Engine light stay on without looking into the issue, here’s a fast fact that’ll wake you right up: if the source of your Check Engine light is a faulty O₂ sensor, you could be reducing your gas mileage by up to 40%, or $900 a year of extra fuel costs, according to Business Insider.

An oxygen sensor monitors unburned oxygen from the exhaust and controls how much fuel is burned. Most cars have between 2 and 4 oxygen sensors. Although the fix (below) is simple, if you don’t address the problem immediately it can lead to a broken catalytic converter, which costs about $2,000 to fix.

Fix it: Dust off your car manual and learn the location of your O₂ sensor. Unclip the sensor and replace it with a new one.

Car gas cap

Car gas cap

Loose gas cap

The second most common instigator of the Check Engine light, a loose gas cap allows gas vapors to leak out and will throw off the entire fuel system.

Once the fuel system is thrown off your fuel economy goes down (by .5%, according to Car MD) and emissions go up.

Fix it: Inspect your gas cap! Retighten it if necessary and look for any cracks. If you do find cracks, visit an auto parts store and purchase a new cap – at $3, this is an easy and cheap fix.

Catalytic converter

Catalytic converter

Catalytic converter failure

We don’t recommend pulling up your sleeves and getting under the hood for this one. Usually a broken catalytic converter is the result of ignoring some other problem – most frequently a faulty spark plug or oxygen sensor.

When broken, catalytic converters stop converting carbon monoxide into less harmful emissions. Signs that your catalytic converter is failing include lack of acceleration when you push the gas and bad gas mileage.

Fix it: We recommend you address the problem’s source first (ie. replace the spark plug or oxygen sensor), but if you have no other choice, prepare to drop no less than four figures at the shop to replace your catalytic converter.

Car mass airflow sensor

Car mass airflow sensor

Broken mass air flow sensor

Similar to an orchestra conductor, the mass airflow sensor directs the entire operation (at least when it comes to air flow). The mass air flow sensor adds the proper amount of fuel based on the amount of air coming to the engine.  Unrepaired mass air flow sensors can reduce gas mileage as much as 25%.

Unreplaced broken mass air filter sensors can cause your car to stall and reduces gas mileage.

Fix it: Nip the problem in the bud. Regularly change out your air filters or face a $400 fee at the shop for mass air flow sensor replacement. If you live in an area that’s dry and dusty you should pay extra attention to changing your air filter – at least once a year is best.

Spark plug

Spark plug

Spark plug failure

Small but crucial, the spark plug  fires the explosive air and fuel mixture in your car’s internal combustion engine while also sealing the combustion chamber. Like most car parts, spark plugs wear out naturally over time and will need to be replaced.

Fix it: Replace your spark plug yourself or take your car to the shop. Luckily, spark plugs are easily accessible from the hood of your car and going the DIY route will cost less than $10.


For more useful car care tips, check out our blog on 8 things teen drivers – and really all drivers – should know or our post on the importance of a brake inspection. And to learn more about 2nd to None Service, check out our website.


~ by 2ndtononeservice on July 12, 2013.

One Response to “The five most common reasons your Check Engine light turns on”

  1. […] a car that is noticeably out of tune or has failed an emission test can improve gas mileage by 4%. Fixing a serious maintenance problem, such as a faulty oxygen sensor, can improve your mileage by as much as 40% (gas savings: […]

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