Top 10 Reasons Why Your Check Engine Light is On

Jason Siu
by Jason Siu

If your check engine light is on, there’s a good chance it’s for one of these 10 reasons.

CarMD has released its 2016 Vehicle Health Index, documenting the top most common check engine repairs in 2015. This year’s Index statistically analyzes 1,019,904 repairs that were uploaded to the CarMD diagnostic database from January 1, 2015 to December 31, 2015. All 50 U.S. states, including the District of Columbia, are represented in the Index. The average repair costs are based on parts and dealer list plus 10 percent markup, while labor rates are procured from several sources including the Undercar Digest National and Regional Hourly Shop Labor Rate reports.

SEE ALSO: What Does That Light on My Dash Mean?

CarMD also highlighted that vehicle age affects the type, cost and percentage of check engine light repair incidents with model year 2006 vehicles accounting for more than 10 percent of cars needing repairs last year. They also had the highest average repair cost at $399. Brand new 2016 model year vehicles accounted for only 0.01 percent of repairs, with an average cost of $205, which was usually covered under warranty.

10. Replace Evaporative Emissions (EVAP) Purge Solenoid

Up from the 15th spot in last year’s Index, the purge solenoid is part of the car’s EVAP system and helps control how much fuel vapor escapes into the atmosphere from the car. The purge solenoid is controlled by the engine control module or powertrain control module and operates on a duty cycle and could be partially left open. The average cost to have the purge solenoid replaced in 2015 was $184.66 and accounted for 2.27 percent of all repairs last year.

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9. Replace Evaporative Emissions (EVAP) Purge Control Valve

The purge control valve is part of the car’s EVAP system, and when the engine is running and fully warmed up, the engine computer gradually opens the purge valve to allow some of the fuel vapor to be moved from the charcoal canister to be burned in the engine. If the purge flow is monitored as more or less than expected by the engine’s computer, the check engine light will come on. The purge valves can also get stuck, causing a need to be replaced. The average cost to replace the purge control valve in 2015 was $168.11 and accounted for 2.83 percent of all repairs.

8. Replace Spark Plug Wire(s) and Spark Plug(s)

Responsible for igniting a car’s air/fuel charge or mixture, faulty spark plug wires and/or spark plugs can cause a “misfire,” reducing gas mileage and potentially damaging the car’s catalytic converter. In areas with cold weather, fuel doesn’t vaporize as easily, so droplets can form and foul a spark plug. Spark plugs are relatively cheap to replace yourself, costing around $10, although the average cost to replace spark plugs and spark plug wires last year was $331 with $179 in labor and $151 in parts. The issue accounted for 3.42 percent of all repairs last year.

Buyers Guide: The 7 Best Spark Plugs and Everything You Need to Know About Them

7. Replace Mass Air Flow Sensor

The mass air flow sensor is in charge of metering the air coming into your car’s engine, helping determine how much fuel to inject into the engine. When the mass air flow sensor is malfunctioning, fuel economy can drop anywhere from 10 percent to 25 percent. Last year, replacing the mass air flow sensor cost an average of $382.36 and accounted for 3.49 percent of all repairs.

6. Replace Ignition Coil(s)

Ignition coils are used to provide an ignition source for the fuel to combust in an engine. By taking the battery’s 12-volt current, ignition coils step it up to ignite the spark plugs. Like spark plug wires, several conditions can result in the failure of ignition coils, including faulty spark plugs, high engine bay temperatures and age. Vehicle owners should pay special attention since ignition coil failure could result in more issues such as damaging the car’s catalytic converter. In 2015, the average repair cost to replace ignition coils was $236.32 and accounted for 3.69 percent of all repairs.

5. Replace Thermostat

The thermostat is responsible for regulating the engine coolant temperature to warm and cool to ideal “operating temperature.” It essentially opens and closes as needed to regulate temperature and when a thermostat fails, if often gets stuck open. If the vehicle’s computer doesn’t see the engine coolant temperature rise to operating temperature within a certain period of time, it will illuminate the check engine light. A car’s thermostat can rust and fail if the coolant isn’t changed at recommended mileage intervals or if the vehicle is subjected to extreme temperatures. Last year, replacing a thermostat had an average repair cost of $210.81 and accounted for 3.70 percent of all repairs.

4. Tighten or Replace Fuel Cap

Fuel caps proved troublesome in 2014, accounting for 7.10 percent of all repairs. Last year, it dropped to fourth place, accounting for only 3.84 percent of repairs. Missing or damaged fuel caps can cost time and money, causing the check engine light to illuminate. If left unchecked, a gas cap issue can cause reduced fuel economy and harm the environment. Luckily it’s a cheap fix, with the average cost to repair coming in at $15.31.

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

Serving as proof that ignoring faulty spark plugs can eventually cause more issues, replacing ignition coils and spark plugs was the third most common check engine light repair in 2015. It accounted for 6.19 percent of all repairs last year with an average repair cost of $390.67.

2. Replace Catalytic Converter

Accounting for 6.97 percent of repairs in 2015, replacing a vehicle’s catalytic converter proved costly with an average repair cost of $1,153.49. In most cases, a catalytic converter won’t fail unless a misfire occurs, which can be caused by ignoring a faulty spark plug, ignition coil or engine mechanical problem.

1. Replace Oyxgen Sensor

The most common car repair in 2015 was replacing the oxygen sensor, making up 7.01 percent of all repairs last year. The O2 sensor is extremely important to a car’s engine performance as well as to the environment, measuring the amount of unburned oxygen in the exhaust to tell a car’s computer when there is either too much, or not enough fuel as compared with oxygen for ideal operation. O2 sensors can fail prematurely for a variety of reasons, including lack of maintenance such as ignoring oil changes or engine contamination from internal coolant leaks. If ignored, a faulty O2 sensor could result in as much as a 40-percent reduction in gas mileage. The average cost to replace an oxygen sensor last year was $249.92.

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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  • Thomasg49 Thomasg49 on Apr 29, 2017

    disconnect the ground post for 30 seconds...The Check Engine light should reset and stay off..easy fix...If the light comes back on right away...there is a problem,,,autozone will code it for for you..99% of the time, its a O2 sensor...Most cars today have at least 2....One before the catalytic converter...and One after ...understanding the code reading witch one is bad..

    • Manny Manny on Aug 09, 2018

      usually it is the first one before cat converter or closest to engine

  • Manny Manny on Aug 09, 2018

    i have an 04 expedition engine turns starts after 2 minutes shuts off by itself can fuel pump be culprit ??? no service engine light on or nothing