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You're driving along, everything seems to be fine, and suddenly -- Oh no! That light just came on, warning that your engine needs attention.

But what exactly does it mean? And how much will it cost you?

Don't panic -- you may need to spend only about $25. Then again, you could be in for a repair bill in the hundreds, according to CarMD's latest analysis of data on check-engine issues.

Here are 2018's most common reasons for those warnings, counting down to the most frequent problem. We've included the average repair costs, including parts and labor.

10. Replace your thermostat

sad woman standing on the road by the broken car in the middle of nowhere. smoke coming out the engine. Help needed. Car service. Tow service.
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If you have a bad thermostat, your car could overheat.

Average repair cost: $225

The thermostat helps regulate the temperature of the engine so it doesn't overheat. At least that’s how things are supposed work in a perfect world.

The thermostat needs to open and close to operate properly, but sometimes it gets stuck open.

When this happens, the car’s computer can’t tell what the temperature of the engine coolant is. It flips the check-engine light on to ask for help. Rust and extreme temperatures are common causes of thermostat failure.

9. Replace your fuel injectors

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

Average repair cost: $447

Fuel injectors do exactly what their name implies: They inject fuel into the engine. When they’re working correctly, they spray just the right amount of fuel into the cylinder in a fine mist.

Fuel injectors have become incredibly refined and precise. A failing fuel injector can lead to reduced fuel economy, 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 better for fuel economy but are more prone to failure.

8. Replace your EVAP purge solenoid

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An EVAP purge solenoid is easier to understand than it sounds.

Average repair cost: $151

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

It's formally known as your car's "evaporative emissions (EVAP) purge solenoid." Here's what it is: A gadget that helps regulate the amount of fuel vapor that your car emits into the atmosphere.

The check-engine-light may come on as a result of the purge solenoid being left partially open.

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7. Replace your ignition coils

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The ignition coils help start the engine.

Average repair cost: $218

An ignition coil takes energy from the car's battery and amplifies the bejeezus out of it. It then sends it 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 that you have up to 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.

6. Replace your mass airflow sensor

Car need fuel
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When your mass airflow sensor needs attention, your car might be using too much fuel.

Average repair cost: $340

The job of the mass airflow sensor (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.

Too much fuel being injected is bad for the environment, and not enough puts stress on the engine. When the MAF is malfunctioning, your fuel economy can decrease as much as 25%.

5. Replace EVAP purge control valve

Refilling fuel view from inside of gas tank of a car

Average repair cost: $146

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 is stuck and needs replacing can trigger the check-engine light.

4. Tighten or replace your fuel cap

Woman opening fuel filler at the gasoline station
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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 fuel 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.

3. Replace your catalytic converter

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

Average repair cost: $1,271

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 harmful gases into less harmful ones through chemical reactions.

Catalytic converters usually don't fail on their own. When they go out, it's often the result of a faulty spark plug that's been ignored for too long.

2. Replace your ignition coil and spark plugs

An auto mechanic shows a damaged and worn spark plug as he performs a tune up.
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Average repair cost: $367

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 of these guys goes out, the other usually isn't far behind.

It’s best to catch these problems early, when a spark plug goes out. Otherwise, you may find yourself in store for more than one repair.

1. Replace your oxygen sensor

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Average repair cost: $238

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.

A faulty O2 sensor is the most common reason when a car's check-engine light goes on, according to CarMD. A bad sensor will reduce fuel economy and put more wear on your vehicle.

Obey your check-engine light! It's there to save you money and stress.

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