Top 10 check-engine-light car repairs

Top 10 check-engine-light car repairs © Tatiana Chekryzhova/

Replacing oxygen sensor(s) © Ruslan Kudrin/

No. 1: Replacing oxygen sensor(s)

Average repair cost: $249.92

For the sixth year in a row, oxygen sensor malfunctions remain the most common car repair, accounting for 7.01% of repairs in 2015.

The so-called O2 sensor can fail as a result of neglecting car repairs, engine issues (such as internal leaks or burning oil) or simply from using gas with a higher concentration of ethanol. Because the sensor works with the car's on-board computer to create the fuel-to-air ratio, a malfunction can result in as much as a 40% drop in fuel economy and can affect performance. The average cost of the repair decreased $10 from the prior year.

Failing catalytic converter(s) © Einar Muoni/

No. 2: Failing catalytic converter(s)

Average repair cost: $1,153.49

For the second year in a row, catalytic converter troubles are the second most common issue drivers face.

This is primarily because these parts won't fail unless there is an underlying problem. It represents 6.97% of repairs in 2015, an increase of more than 1% over the prior year. This is the most expensive repair in the top 10 by far.

Faulty ignition coil(s) and spark plugs © Pylj/

No. 3: Faulty ignition coil(s) and spark plugs

Average repair cost: $390.67

Up from fourth place from the year before, this accounts for 6.19% of repairs in 2015 -- nearly double from 2014, when it was 3.58% of repairs.

Because ignition coils use the current from the car battery to ignite the spark plugs, ignoring spark-plug problems can result in a faulty ignition coil (or coils). These parts also fail over time due to high under-hood temperatures.

Loose fuel cap ©

No. 4: Loose fuel cap

Average repair cost: $15.31

Drivers are getting the message that the check-engine warning can be triggered by a gas cap that's loose, missing or damaged. It also allows gas to evaporate. This problem accounts for 3.84% of repairs, compared to 7.1% in 2014.

Tightening a loose gas cap is free, but expect to spend about $15 if the cap needs to be replaced.

Replace thermostat |

No. 5: Replacing thermostat

Average repair cost: $210.81

Up from eighth place last year, this represents 3.7% of repairs.

A car's thermostat regulates the engine's temperature by opening and closing to regulate the flow of coolant. If the coolant isn't changed as recommended, or the car itself is driven in extreme temperatures or in an acidic environment, corrosion can occur, causing it to fail prematurely.

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Faulty ignition coil(s) © Winai Tepsuttinun/

No. 6: Faulty ignition coil(s)

Average repair cost: $236.32

Replacing the ignition coil or coils (some cars have more than one) accounts for 3.69% of the repairs in this year's study, down from fifth place last year.

Ignition coils can malfunction due to high under-hood temperatures and the overall age. Pay attention to problems, as ignition-coil failures can cause other problems, including the pricey catalytic converter.

Replace mass air flow sensor © Winai Tepsuttinun/

No. 7: Replacing mass air flow sensor

Average repair cost: $382.36

A malfunctioning mass air flow sensor, or MAF, accounts for 3.49% of repairs, according to CarMD's latest study.

Problems with the MAF can result in decreases in fuel economy between 10% and 25%, since it's the MAF that measures the air coming into the engine and is what calculates how much fuel to inject. In addition to this repair dropping one place in rank, costs have also declined from $409 to $382.

Bad spark plugs and spark plug wires © Pylj/

No. 8: Bad spark plugs and spark plug wires

Average repair cost: $331.13

Dropping one spot from last year, this represents 3.42% of repairs.

Replacing a spark plug can cost just $10 if you do it yourself, but drivers spend $331 to have spark plugs and their wires replaced by a technician. When these items fail, they can cause the engine to misfire and your gas mileage to decline. Ignoring the problem can result in permanent damage to your car's expensive catalytic converter.

Malfunctioning evaporative emissions (EVAP) purge control valve | Photo courtesy of

No. 9: Malfunctioning evaporative emissions (EVAP) purge control valve

Average repair cost: $168.11

Up from No. 14 in 2014, this represents 2.83% of check-engine-related repairs.

This valve prevents vapors from your car's gas tank from being released into the atmosphere and instead channels them into a charcoal canister to be burned in the engine. Over time, the valve sticks and often needs to be replaced.

Replace evaporative emissions (EVAP) purge solenoid | Photo courtesy of

No. 10: Replacing evaporative emissions (EVAP) purge solenoid

Average repair cost: $184.66

Jumping from No. 15 in CarMD's study last year to No. 10 this year, this represents 2.27% of all repairs.

Like the purge control valve (at No. 9), the solenoid is part of the car's EVAP system, controlling how much fuel vapor escapes into the atmosphere. It is controlled by the engine control module or powertrain control module and typically needs replacing when it starts to fail.

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