TECH: Top 10 Reasons Why Your Check Engine Light Is On

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5 reasons check engine light

Wondering what it’s going to cost you when your car’s check engine light comes on? The 2016 CarMD Vehicle Health Index is going to give you a pretty good indication based on more than 1 million cases when that scary dashboard light became illuminated.

What’s good about the report is it breaks down the average cost of repairs; what the most common repairs are; and, how much you’re going to pay in different regions of the country among other useful information.

The CarMD Vehicle Health Index reports on the most common check engine light-related problems, repairs and associated repair costs. It has been published since 2011 but has data going back much further.

David Rich, CarMD’s technical director, tells how to use the health index as part of your vehicle’s maintenance program. He said, “This CarMD Vehicle Health Index reminds drivers to address check engine light warnings early to help reduce the likelihood of additional repairs down the road, maximize fuel economy and help make sure your car isn’t harming the environment.”

Rich notes that, if ignored, a simple spark plug failure can snowball from a $50 part into a $400 repair that entails spark plug and ignition coil replacement, or worse. How much worse? One unfortunate owner needed to replace his vehicle’s engine for a cost of $7,800. CarMD doesn’t detail what kind of car it was.

Top 10 reasons for the Check Engine Light

OK, so you’re driving down the road and your check engine light comes on. Odds are good it’s one of the 10 following reasons, according to the CarMD report. Now for the bad news. The cheapest fix is $15. The most expensive – among the top 10 reasons – is replacing the catalytic converter at $1,153.

According to CarMD, the following are the 10 most common check engine light-related car repairs and associated average repair costs. left unrepaired, each of these problems will negatively impact fuel economy and harm the environment:

  1. Replace oxygen sensor – $249
  2. Replace catalytic converter – $1,153
  3. Replace ignition coil(s) and spark plug(s) – $390
  4. Tighten or replace fuel cap – $15
  5. Replace thermostat – $210
  6. Replace ignition coil(s) – $236
  7. Replace mass air flow sensor – $382
  8. Replace spark plug wire(s) and spark plug(s) – $331
  9. Replace evaporative emissions (EVAP) purge control valve – $168
  10. Replace evaporate emissions (EVAP) purge solenoid – $184

You know what’s reassuring? Some of these repairs sound really expensive. Replacing a purge solenoid sounds like something you would need a home equity loan for but somehow $184 doesn’t seem all that bad.

CarMD’s background

For 20 years now, on-board diagnostics (OBDs) have been standard on all passenger vehicles sold in the United States. They provide (when an electronic device is connected) health and safety information. CarMD says they capture date on roughly 80% of a vehicle’s systems, and is currently installed on more than 85% of vehicles nationwide, including newer hybrids and diesels.

It’s the OBD that is going to engage the check engine light when it thinks something is amiss. Most of us don’t care about vehicle emissions but you should at least care about engine repair costs.

Since 2001, CarMD has been working with a nationwide network of Automotive Service Excellence (ASE)-certified technicians and Master technicians to build a comprehensive database of OBD-related expert fixes and repair costs.

The vehicle failures come directly from the cars themselves and the recommended repairs come from the professionals who service them. Thanks to that date, CarMD says it is able to provide data on repair costs and trends.

Cost of Repairs Map

Repairs Depend on Where You Live

It’s a sign of where the economy is going that labor costs didn’t rise on average when it came to car repairs. The good news is average repair costs from 2014 to 2015 dropped 4 percent.

Parts, however, showed an increase of 1.5 percent. That’s more than double the increase in the Consumer Price Index. It pegged inflation at .7 percent from December 2014 to December 2015.

Car repair costs were down across each U.S. region with the Northeast experiencing the biggest drop – down 6.5 percent to $391.17. The West is the most expensive region with an average cost of $403.42, down 4.7 percent. The South comes in at $388.64, a drop of 3.1 percent.

It costs an average of $364.61, a drop of 2.9 percent in the Midwest. That’s the lowest region in the country.

How Old Is Your Car?

Vehicle age also affects frequency, cost and type of repairs. CarMD broke repair costs out for models from 1996, 2001, 2006, 2011 and new.

The most common reason
a check engine light comes on in 
a brand new vehicle is a loose gas cap (46 percent of recommended repairs for MY2016 vehicles). The most common check engine light on a 2006 vehicle is the catalytic converter (10 percent of repairs). Model year 2006 vehicles had the highest average repair cost ($399).

The most common repairs on 2011 model year vehicles were replace ignition coil and/or spark plugs for a cost of around $390. Both 2001 and 1996 model-year vehicles are going to need their oxygen sensors replaced most frequently for a cost of $249.

$15 to Tighten a Gas Cap?

tighten gas capOK, so it might seem ludicrous that it’s going to cost you $15 on average to have your gas cap checked as the culprit for a check engine light. That’s driven by the actual cost of the gas cap more than anything and, yes, it’s ludicrous that people are going to need their gas cap replaced when the check engine light comes on. A good service center is just going to twist your gas cap off and twist it right back on for free.

The most expensive repair seen in
2015 by CarMD’s network was “replace engine,” costing $7,800. However, evidencing the notion that drivers should not panic when their car’s “check engine” light comes on, some of the least expensive repairs include tighten or replace gas cap, correct transmission fluid level, and replace radiator cap. These fixes can cost as little as $15 or less.

However, if your car needs these horrible repairs you might want to panic just a little bit.

Looking for a new or used car? Check out’s local search.

Keith Griffin

Keith Griffin

Keith Griffin is an automotive columnist for the Las Vegas Review Journal and, as well as a reviewer for the Boston Globe, and - most importantly - dad to two great girls.