Top 10 Reasons Why Your Check Engine Light is On

Top 10 Reasons Why Your Check Engine Light is On

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.

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.

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.

  • smartacus

    Have i got a check engine light story for you.

    Coincidentally; last week i was about to be given a smart car loaner when i brought in my vehicle to MB West Palm Beach for an alignment.
    Their brand new 2016 smart car loaner already had a check engine light!!!
    Naturally; my first inclination was to check the gas cap to no avail.

    OK they insisted not loaning me a car with the CEL so they set me up with a Mustang Convertible loaner car which had no check engine lights, went hella faster, fit the baby seat in the back, probably fits an entire dresser with the roof down, aaand averaged 32MPG on 87 regular with a very light foot.

  • Martin Plamondon

    Mass airflow sensor was my problem a couple of years ago, I didn’t change it, bought the sensor cleaning product for around 10$ and it did the job, try this before spending over 300$ for a replacement.

  • Lennie Pike

    The most common repair is go to an auto parts store, borrow a scanner and clear the code. By far the most common code is the loose gas cap. The other common code is one for whatever sensor the goof at the cheap oil change forgot to plug back in when they tried to sell you an air filter.

  • Roy Pacheco

    Try filling up with Shell High Octane and 9/10 the ‘check engine’ light might go out!

  • Big Boy

    There is a system on most if not all cars and trucks that keeps a track on mileage and the oil condition when the limit is reached the engine light will com on.
    When the dealer does an oil change it is a simple matter to reset this if he does not do that the oil may be clean but the mileage will activate the light.
    a good mechanic will reset it free of charge a bad one will try and sell you one or more of the ten items mentioned in this article.
    Jason Slu who wrote this article should have made this the first thing to be checked but no he must be one of the bad mechanics trying to sell you something you don’t need

  • slamfist

    I’ve never even heard of that. I’ll have to look into that.

  • Bloody Hell

    A car’s thermostat can rust and fail if the coolant isn’t changed …………………I have been driving and servicing my own cars and those of friends and in 65 years I have chanced many thermostats….I have found them stuck.. open/closed, some bent/broken, plugged with scale…..BUT I have NEVER found one RUSTED!!!!!! ever…….I call BS on that!

  • C Jimmy D Collins

    I change my own oil most of the time and my owners manual tells how to do the reset which i do and it’s easy to do .

  • Evil Larry

    I agree with you on that. The only times I’ve seen a thermostat rust is at junk yards where there was obviously no coolant in the engine for quite some time. These so called tech articles often are based on fear and half truths.

  • WOOD


  • Ede

    My car has its check engine light on whenever it wants an oil change done. This light will come on hundreds of miles before I should get one done so I have to call bs on this as I’m not the only person with this make and model that has this occur.

  • Mike Miller

    I have. Customers changed the Coolant with Strait Water and it fucked the Entire cooing system Rusted the thermostat’s. Not everyone has a clue how to take care of there vehicles

  • Roy Pacheco

    Stop using Petro Canada Gas and switch to Shell Premium ..just on full tank!

  • Eric W Diaz

    I bought an old car off someone and the radiator was filled with plain old water and you bet it was rusty. The radiator cap had rusty sludge and the water was very clumpy as well as the thermostat. I had to have the entire coolant system replaced… hoses, water pump, rusty thermostat, radiator etc.

  • Benny Boggess

    My Chrysler’s change oil indicator light comes on between 3000 and 3500 miles after the oil has been changed. My service / owners manual says to charge oil and filter at 8000 miles. So when it comes on at 3000 miles I reset it and then change oil and filter when it comes on the second time.

  • Benny Boggess

    Brass doesn’t rust

  • tuffee

    My truck,s check engine has been on since I got it 6 years ago

  • Michael Wright

    Do you ever get it inspected?

  • tuffee

    Don,t have to where I live but Ive tried just about everything stays on

  • Bernie Malinowski

    Where did they get those prices???

  • Réjean Drouin

    Reason #1 : Its a Volkswagen
    Reason #2 : It’s a Audi

  • Greg Macdonald

    who has a coil in their car anymore

  • Ray Hinderscheit

    Not the old fashioned traditional coils. Now every cylinder has its own coil. They are called coil packs.

  • Roger Epps

    My Toyota’s light is set to come on every 5,000 miles to remind me to get the oil changed. Try shutting it off and see if it comes back on at 5,000 miles. Auto Zone will turn it off for no charge

  • Patric Morrison

    I own a 2004 Chevy Malibu Classic. Every so often the “check engine” light comes on. It goes off in a couple of days. However, if the light keeps flashing, It is a serious problem that needs immediate attention.

  • Thomasg49

    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..

  • Roy Ellithorpe

    I hope you got kissed.