The 10 Most Common Reasons Your Check Engine Light is On

Jason Siu
by Jason Siu

The check engine light can be a bane or boon when it comes to owning a car, especially considering hundreds of things could trigger it.

Does that mean the second the check engine light comes on you need to panic and head to the closest automotive repair shop? You may be surprised by some of the issues that trigger the light and just how inconsequential they may be. Nonetheless, if your vehicle is over five years old, it’s always better to err on the side of caution, especially when you’re unsure what is going wrong.

CarMD has published its 2015 Vehicle Health Index and has listed the top 10 most common reasons your check engine light is on and the vehicle repairs associated with it. It could give you an idea of just how deep you’ll have to dig into your pockets if that light illuminates on your dash.

10. Replace Fuel Injectors

Coming in at 10th place is replacing fuel injectors, which has an average total repair cost (parts and labor) of $554.36. According to the report, replacing fuel injectors made up 1.75 percent of repairs done in 2014 and is actually new to the list for this year. Common reasons why fuel injectors need to be replaced include clogging from fuel tank debris resulting from driving frequently with the fuel light on, fueling up with low-grade gas or putting off regular maintenance such as a fuel filter replacement.

9. Replace Exhaust Gas Recirculation (EGR) Valve and Clean All EGR Ports

With an average total repair cost of $349.88, replacing your exhaust gas recirculation (EGR) valve and cleaning all the EGR ports can be costly, but is vital to ensuring your vehicle performs for a long time. The service made up 1.95 percent of 2014 repairs.

8. Replace Thermostat

Considering the thermostat in a vehicle works harder than many other components in the engine bay, it’s no surprise that it was the eighth most common check engine vehicle repair last year. More susceptible in high-heat regions, replacing your thermostat can run you an average of $213.20 and was 2.18 percent of 2014 repairs.

7. Replace Spark Plug Wires and Spark Plugs

Making up 3.28 percent of 2014 repairs, replacing spark plug wires and spark plugs is considered standard maintenance on a vehicle. Let it go too long and your car could throw a check engine light and you’ll be out an average of $361.56. If left alone, a spark plug can cease, eventually causing failure with the ignition coil and catalytic converter, at which point you’re just paying more to fix your car.

6. Replace Mass Air Flow (MAF) Sensor

Replacing the mass air flow (MAF) sensor is the sixth most common check engine light repair in the U.S., but was actually the third most common in the West. The MAF is more vulnerable in dry, dusty summer road conditions or snow, salt and ice and can be a costly replacement at an average repair cost of $408.62. According to the 2015 Vehicle Health Index, replacing the MAF made up 3.54 percent of 2014 repairs and can actually be avoided by keeping your car’s air filter clean, which can cost as little as $30.

5. Replace Ignition Coil(s)

Having to replace an ignition coil can be caused by ignoring maintenance on your spark plugs and spark plug wires, at which point you’ll be paying another $247.39. The report states that the issue comprised 3.57 percent of 2014 repairs and is the fifth most common check engine light repair.

4. Replace Ignition Coil(s) and Spark Plug(s)

It’s no surprise that having to replace the ignition coil and spark plugs are grouped together as the next offender, seeing the average total repair cost increase to $413.92. In many cases, once the ignition coil goes bad, it’s likely that faulty spark plugs were the culprit. This issue made up 3.58 percent of 2014 repairs.

3. Inspect for Loose Fuel Cap and Tighten or Replace as Necessary

It might seem trivial, but having a loose fuel cap will make the check engine light come on. So if you’ve recently filled up your gas and noticed the check engine light illuminating, you might want to save yourself some time and trouble and make sure your gas cap is fully tightened. A mechanic will likely charge you no fee to fix the issue, considering it had an average total repair cost of just $0.01 but it did make up 5.4 percent of 2014 repairs.

2. Replace Catalytic Converter(s) with new OE Catalytic Converter(s)

Having to replace a catalytic converter is common, especially with vehicles that are older. But you can expedite the issue by not properly maintaining your car or keeping fresh spark plugs in when necessary. It’s an expensive fix too, with some vehicles costing thousands of dollars for new catalytic converters – we’re looking at you, German luxury cars. The total average repair cost for the issue last year was $1,157.27 and made up 5.89 percent of 2014 repairs.

1. Replace Oxygen Sensor(s) (O2s)

The most common issue that triggers the check engine light last year was having to replace the oxygen sensors (O2) in the vehicle. Fortunately it’s one of the cheapest fixes on the list as well, with an average total repair cost of $259.30. Unfortunately, a faulty oxygen sensor can cause your vehicle’s ECU to do crazy things, such as dumping in more fuel and making the engine run rich. As a result, you could lose as much as 40 percent of your fuel economy from a bad O2 sensor. As the most common check engine light repair in 2014, replacing the oxygen sensors made up 7.10 percent of 2014 repairs.

Jason Siu
Jason Siu

Jason Siu began his career in automotive journalism in 2003 with Modified Magazine, a property previously held by VerticalScope. As the West Coast Editor, he played a pivotal role while also extending his expertise to Modified Luxury & Exotics and Modified Mustangs. Beyond his editorial work, Jason authored two notable Cartech books. His tenure at saw him immersed in the daily news cycle, yet his passion for hands-on evaluation led him to focus on testing and product reviews, offering well-rounded recommendations to AutoGuide readers. Currently, as the Content Director for VerticalScope, Jason spearheads the content strategy for an array of online publications, a role that has him at the helm of ensuring quality and consistency across the board.

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2 of 4 comments
  • Jadane Jadane on Sep 07, 2017

    The most common reason the light comes on is that the tape you keep over it came off.

  • Kristen Jay Kristen Jay on Oct 18, 2017

    My 2010 Accord's check engine and oil light come on.Maybe I'll ask the mechanic if he can just disconnect the lights if it will cost to much to fix