Check Engine Light On? Here's What It May Be Telling You

  • Oct 31,2016
Yellow and black close up of car's check engine light on Yellow and black close up of car's check engine light on

There's just something about things that are mysterious, unsettling, and overall spooky, that can be appealing...except when it comes to our vehicles. When the check engine light flashes on, even the most experienced drivers start feeling as nervous as Cinderella at midnight. Why? Since a check engine light signals a variety of issues ranging from "no biggie" to "whoa Nelly," it can be difficult to know what comes next, kind of like turning the corner in a haunted house. When it comes to your car's check engine light, save the screams for movie night and check out the top five reasons this little light might be on!

Issue #1: Faulty Oxygen Sensor

Snuggled into your vehicle’s exhaust pipe is a little sensor that constantly tests your car's exhaust system to determine how much oxygen is in it. The sensor sends its findings to your car's computer, which then turns around and uses this data to maintain the perfect air to fuel mixture for optimal engine performance.

Because symptoms of a bad oxygen sensor aren't well known or obvious, drivers tend to learn about this issue only when their check engine light pops on. If you're in tune with your gas gauge though, you may notice that something is off if you find yourself having to fill up more often. A major maintenance problem like a faulty oxygen sensor can decrease your fuel economy by up to 40%, says the U.S. Department of Energy.

Fortunately, this one's an easy fix: all you need to do is replace the sensor. According to, this repair costs just under $200 to fix. Ignore it, though, and you could wind up with catalytic converter issues and a repair cost of over $1,000. Yikes!

Issue #2: Faulty Mass Airflow Sensor

The mass airflow (MAF) sensor is king of fuel delivery and spark timing. This device uses a live wire to measure airflow, which an engine computer then processes to make all sorts of important internal adjustments. When the MAF sensor isn’t working properly, it can lead to stalling, decreased fuel economy, and rough idling, reports CarsDirect.

A MAF sensor can be removed and cleaned, or fully replaced, but try changing your air filter first. Your car's air filter directly impacts the MAF sensor, as it keeps dirt, dust, and other gunk you encounter on the road from entering your engine. A filthy air filter means that dirt could be unnecessarily accumulating on the MAF sensor, reducing its effectiveness, and triggering your check engine light.

Issue #3: Worn Spark Plugs

Spark plugs are like Zeus’s little bolts of lightning! Small but powerful, these plugs produce the spark of electricity that makes it possible for your car to start. To do so, the spark even has to jump across at little gap! As the plugs age, the extreme conditions to which they are continually exposed can cause damage in the form of fluid build-up and an expanding gap for the spark to cross. Left un-replaced, worn spark plugs can cause ignition problems, a loss of fuel economy, or damage to other internal systems. Not to mention, without a spark, your car won't start. Now that's pretty important business!

This fix isn't as speedy as some other ones on this list, but it's a pretty important one. In order to replace your vehicle's spark plugs, the technician has to wait until the car is cold and in some cases, disassemble parts of the vehicle in order to reach the spark plugs. Because a vehicle drop-off is typically required for this repair, you may want to schedule a spark plug replacement ahead of time.

Pro Tip: So your check engine light is on, but not 24/7. Should you worry? Here's the scoop. Your car is definitely telling you there's an issue. It's not a super serious one (yet) but it's best not to ignore it. Having the issue investigated and resolved sooner, rather than later, could leave you with a much smaller repair bill at the end of the day.

Issue #4: Damaged Catalytic Converter

If fright night rolled around, no one egged your house, and you still smell rotten eggs, you might have a busted catalytic converter, notes CarsDirect. This is where your engine's fumes (carbon monoxide, nitrous oxide, and hydrocarbons) go to be converted into water and carbon dioxide. Beyond tainting your car with a noticeably nasty scent, a damaged converter can lead to a decrease in fuel efficiency and driving speeds. Damage, typically in the form of blockage, often occurs because faulty plugs or sensors have gone untreated for too long.

Catalytic converters tend to be pretty reliable, but when they do break down it's kind of a big deal. Help keep yours from breaking down by keeping up with your car's regularly scheduled maintenance and fixing the little things (like faulty plugs and sensors) before they fail completely.

Did you know? If you drive an antique vehicle or souped up hot rod, your car probably doesn't have a catalytic converter. Then again, it probably doesn't have a check engine light either!

Issue #5: Loose, Missing or Damaged Gas Cap

If thoughts of expensive repairs have given you the creepy crawlies, there’s still hope: your check engine light could be reacting to a problem with your car’s gas cap.

Repairs range from tightening the cap (quick and free) to replacing it (still only around $15). Either way, the gas cap is definitely worth keeping an eye on—when it’s loose, missing, or damaged, gas evaporates, which sends you to the pumps more often. While gas prices aren't necessarily giving anyone goosebumps right now, they very well could! We've all seen how gas prices like to go down...but also up. A quick twist on the cap could save you serious dough in the long run.

When Your Car Works, Everything Works

When your car runs smoothly, your tank is full, and your car's dashboard isn't lit up like a porch full of jack o' lanterns, driving feels just plain awesome. If your check engine light is keeping you up at night, it's time to make the unknown, known! Visit your nearby Firestone Complete Auto Care location for a free car inspection. Your check engine light could be a sign of seriously spooky engine troubles!

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