• Discounts and special offers
  • Subscriber-only articles and interviews
  • Breaking news and trending topics

Already a subscriber?

By signing up, you accept Moneywise's Terms of Use, Subscription Agreement, and Privacy Policy.

Not interested ?

Here's the latest ranking of the most common reasons for that dreaded dashboard warning indicator, counting down to the most frequent problem. The average repair bills include parts and labor.

What would make a check engine light go on?

10. Replace your thermostat

Hand a man put car thermostat for service
NONGASIMO / Shutterstock
Thermostats can fail due to rust or other issues.

Average repair cost: $245

The thermostat helps regulate the temperature of the engine so it doesn't overheat. The device needs to open and close to operate properly, but sometimes the thermostat gets stuck in the open position.

When this happens, the car's computer can't tell if the engine coolant is reaching the proper operating temperature, and that triggers the check-engine light. Rust, a failure to change the coolant, and extreme temperatures are common causes of thermostat failure.

This problem accounts for 2.4% of all check-engine repairs, and the cost has gone up 8.9% since CarMD's 2018 report.

8. (tie) Replace your fuel injectors

Car mechanic fixing fuel injector at  two camshaft gasoline engine
sima / Shutterstock
Faulty fuel injectors can lead to all sorts of problems.

Average repair cost: $450

Like the name implies, fuel injectors inject fuel into the engine. They have become incredibly refined and precise, and when a fuel injector fails it can result in reduced fuel efficiency, engine misfires and performance issues.

Many vehicle manufacturers have recently changed over to direct injection (DI) fuel systems, which use high-pressure injectors. These are generally better for fuel economy but are more prone to failure.

Injector failure is responsible for 2.78% of check-engine repairs; the cost has inched up 0.7% since the last survey.

8. (tie) Replace your EVAP purge solenoid

Auto Service Business Concept. Pro Car Mechanic Taking Care of Vehicle. Checking Under the Car Hood.
welcomia / Shutterstock
An EVAP purge solenoid is easier to understand than it sounds.

Average repair cost: $154

Huh? Trust us: This one is a lot simpler than its head-spinning name.

The "evaporative emissions (EVAP) purge solenoid" is a gadget that helps regulate the fuel vapor your car spews into the atmosphere. The check-engine light may come on if the purge solenoid can't close.

An EVAP thingy that needs replacing is the reason for 2.78% of check-engine repairs, and the cost has risen 2% in a year. Think a new car may be your ultimate answer? A personal loan can make that happen.

7. Replace your mass airflow sensor

Mass air flow sensor of a car at the air filter housing.
Valokuva24 / Shutterstock
A faulty MAF can cut fuel economy by 25%.

Average repair cost: $341

The job of the mass airflow sensor, or MAF, is to monitor how much fuel is being sent into the engine by the fuel injectors. You want the right amount injected for optimal combustion to take place.

The miniature explosions in the car’s pistons are what propel the car forward. When your MAF has gone bad, your fuel economy can drop by as much as 25%.

A mass airflow sensor that's on the blink is responsible for 3.6% of check-engine repairs. The cost of replacing one has gone up just $1 (0.29%) from what it was in last year's Vehicle Health Index.

6. Replace EVAP purge control valve

Refilling fuel view from inside of gas tank of a car

Average repair cost: $150

The EVAP purge control valve works with the purge solenoid to keep fuel tank vapors from seeping into the atmosphere.

As the engine warms, its computer triggers the valve to open. This allows more fuel vapor to move into the engine, where it can be burned.

A valve that's stuck and needs replacing is the source of 3.61% of check-engine repairs, and the cost has increased 2.7% in a year.

5. Replace your ignition coils

Woman starts the car engine with start-stop button. Modern car interior
plantic / Shutterstock
The ignition coils help start the engine.

Average repair cost: $218

An ignition coil takes current from the car's battery and amplifies the bejeezus out of it. It then sends that energy on over to the spark plugs so they can start the engine.

Some vehicles have only one ignition coil, others have several. If you have a six-cylinder engine, it’s possible you have as many as six ignition coils.

Older cars and those that run hot tend to have their ignition coils fail more frequently. Bad spark plugs also can cause ignition coil issues, which lead to 3.64% of check-engine repairs. The cost has held steady.

4. Tighten or replace your fuel cap

Woman opening fuel filler at the gasoline station
plantic / Shutterstock

Average repair cost: $26

Your car's fuel cap needs to have a tight seal for the engine to burn fuel efficiently. When your vehicle has to burn more gas due to reduced efficiency, the environment suffers.

If your check-engine light comes on and it isn’t obvious what the cause may be, try tightening your gas cap. It may have just gotten jostled loose.

A worn-out gas cap is easy to repair yourself and could save you a trip to the mechanic. Fuel cap problems flip on the check-engine light 4.18% of the time, CarMD says; the cost of a new cap is unchanged.

3. Replace your catalytic converter

combustion fumes coming out of car exhaust pipe
Orlando_Stocker / Shutterstock
The catalytic converter helps to reduce harmful exhaust from your car.

Average repair cost: $1,371

Catalytic converters help reduce air pollution from exhaust. The toxic compounds found in car exhaust include nitrogen oxide, carbon monoxide and hydrocarbons — all harmful to the atmosphere.

Your car's catalytic converter turns these gases into less harmful ones through chemical reactions. When your converter fails, it's often the result of a bad spark plug that's been ignored for too long.

Catalytic converter problems lead to 4.93% of check-engine repairs, and the replacement cost has jumped 7.9% from the last study. You'll want to get a cash-back credit card to save on that big repair bill.

1. (tie) Replace your oxygen sensor

Professional car mechanic working in auto repair service.
kurhan / Shutterstock

Average repair cost: $244

The oxygen sensor, or O2 sensor, measures the amount of unburned oxygen coming out of your exhaust. There's an ideal amount for both the health of your car and the environment.

When a sensor is going bad, it can reduce fuel economy and put more wear on your vehicle.

A faulty O2 sensor is tied for first among the most common reasons a car's check-engine light goes on, and is responsible for 5.81% of the repairs. The typical cost of replacing the sensor has risen 2.5% year over year.

1. (tie) Replace your ignition coil and spark plugs

An auto mechanic shows a damaged and worn spark plug as he performs a tune up.
Joe Belanger / Shutterstock

Average repair cost: $391

This compounded problem shares first place among check-engine issues; it leads to another 5.81% of repairs. The cost is up 6.5%, according to the 2019 CarMD data.

Spark plugs and ignition coils are a tag team under the hood. They work together to help start the car and keep it running.

The ignition coil converts the battery's 12 volts into the thousands of volts needed to spark the spark plugs. When one member of the team goes out, the other usually isn't far behind — and it'll be game over until you take your car to the mechanic.


This 2 Minute Move Could Knock $500/Year off Your Car Insurance in 2024

Saving money on car insurance with BestMoney is a simple way to reduce your expenses. You’ll often get the same, or even better, insurance for less than what you’re paying right now.

There’s no reason not to at least try this free service. Check out BestMoney today, and take a turn in the right direction.

Doug Whiteman Former Editor-in-Chief

Doug Whiteman was formerly the editor-in-chief of MoneyWise. He has been quoted by The Wall Street Journal, USA Today and CNBC.com and has been interviewed on Fox Business, CBS Radio and the syndicated TV show "First Business."


The content provided on Moneywise is information to help users become financially literate. It is neither tax nor legal advice, is not intended to be relied upon as a forecast, research or investment advice, and is not a recommendation, offer or solicitation to buy or sell any securities or to adopt any investment strategy. Tax, investment and all other decisions should be made, as appropriate, only with guidance from a qualified professional. We make no representation or warranty of any kind, either express or implied, with respect to the data provided, the timeliness thereof, the results to be obtained by the use thereof or any other matter. Advertisers are not responsible for the content of this site, including any editorials or reviews that may appear on this site. For complete and current information on any advertiser product, please visit their website.