Is your check engine light on? Here are 10 possible reasons why

The orange, engine-shaped icon on your car dashboard’s instrument cluster is programmed to turn on when the vehicle’s electronic brain detects something is wrong. Don’t panic if you see it — it doesn’t necessarily mean you’ll need to give your next paycheck to the nearest mechanic. Problems that trigger a check engine light are sometimes basic and quick to fix. Don’t ignore it, either, and get it checked out as quickly as possible.

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Here are the 10 most common problems that can trigger a check engine light. Keep in mind that the fault code that caused the light to turn on in the first place is stored in your car’s onboard computer, and you can retrieve it using a simple Bluetooth-powered device linked to an app to get a precise idea of what exactly is wrong under the hood. Doing this on your own will save you money.

Oxygen sensor failure

Oxygen sensor
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The oxygen sensor (sometimes called an O2 sensor) measures the amount of unburnt oxygen in a car’s exhaust system. It sends data to the vehicle’s computer, which uses it to regulate the mixture of air and fuel that enters the cylinders. An engine will keep running even if an O2 sensor needs to be replaced, but it will burn more fuel than usual. In the long run, a bad O2 sensor can damage components like the spark plugs and the catalytic converter. It may also cause a car to fail an emissions test.

On average, a quality O2 sensor will set you back about $175, but the cost of labor will vary greatly depending on the vehicle’s make and model, as well as your geographical location. Finally, keep in mind that most late-model cars have more than one O2 sensor.

Loose fuel cap

Gas cap
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A loose fuel cap is one of the most common reasons why the check engine light turns on. The cap is a crucial part of a car’s fuel delivery system. It notably prevents gasoline fumes from leaving the fuel tank, and it helps keep the whole system under the correct pressure.

If your check engine light turns on immediately after a fill-up, pull over and make sure the cap isn’t loose — or still on your car’s roof. Sometimes the cap needs to be replaced, but it’s not a problem that’s going to hit your wallet hard. Most auto parts stores carry universal-fit gas caps that cost somewhere in the vicinity of $15.

Catalytic converter failure

Catalytic converter failure

The catalytic converter is integrated into a vehicle’s exhaust system. It turns the carbon monoxide generated during the combustion process into carbon dioxide. It’s a fairly simple part, and its failure can often be prevented. That’s good news, because a new one costs between $200 and $600 depending on the make and model. Every late-model car that runs on gasoline has a catalytic converter.

Performing regular maintenance (such as oil changes) on time is key to keeping your car’s catalytic converter in working order. If you live in the city and mostly drive short distances, take your car on the highway every now and then to ensure the catalytic converter doesn’t get clogged. And as always, keep your eyes and ears open for unusual sounds or discolored smoke coming from the exhaust.

Spark plug/ignition coil issues

Spark plug
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Put simply, an ignition coil generates the electricity the spark plugs need to ignite the fuel and air mixture in the cylinders. Classic cars have a single coil, but many modern vehicles use one coil per cylinder. If your ride has a V8 under the hood, you could have eight separate coils. The monstrous Bugatti Chiron has 16. No matter how many you have though, a malfunctioning coil will almost certainly trigger the check engine light, but remember, if your car burns diesel, you have neither ignition coils nor spark plugs.

Speaking of spark plugs, worn or fouled plugs can cause a variety of issues including an engine misfire and hesitation under heavy acceleration. A worn coil can exhibit the same symptoms, and can cause the car to shut off unexpectedly. A quality spark plug costs between $10 and $20, while a coil is generally in the $50 range. Changing your own spark plugs is also easier than it sounds.

Bad spark plug wires

Spark plug wire
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As its name implies, a spark plug wire transfers electricity from the coil to the spark plug. Without it, the fuel and air mixture in the cylinders wouldn’t ignite. The vast majority of cars use a single wire per cylinder, but there are models — notably, some older Mercedes-Benzes — with two spark plugs per cylinder, and consequently two wires.

Symptoms of bad spark plug wires include a rough idle, a noticeable drop in engine performance, and lower gas mileage. Expect to spend about $50 for a set of plug wires.

Mass airflow sensor failure

Mass airflow sensor failure

The mass airflow (MAF) sensor monitors how much air enters the engine. It’s a part of the engine-management system, so your car wouldn’t be able to adjust to changes in altitude without it. Symptoms of an MAF failure include a rough idle, trouble starting, and a sudden change in the position of the throttle pedal. Reduced gas mileage and stalling can also indicate a MAF problem.

A MAF for a late-model car typically costs between $120 and $150.

Issues with an aftermarket alarm

Car alarm
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An aftermarket alarm can wreak havoc on your car if it’s not installed properly. It can drain the battery, trigger the check engine light, or even prevent the vehicle from starting. Then, when you least expect it, it’ll go off in the middle of the night because a leaf from an oak tree fell on the hood.

If the above issues sound familiar, you’ll need to have the alarm fixed, reinstalled, or replaced entirely by a competent mechanic. Getting it done right in the first place might cost a little bit more, but the peace of mind that comes with having a fully functional alarm is priceless.

Vacuum leak

Car vacuum hoses
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Every car has a vacuum system that performs a wide variety of functions. The brake booster is vacuum-operated, and the vacuum system also helps lower harmful emissions by routing the fumes as gasoline evaporates through the engine. If your car’s idle begins to surge or settles at an unusually high rpm, a vacuum leak could be the culprit.

Vacuum hoses can dry out and crack as they age, especially if they’re exposed to intense heat or extreme cold. This is the most common cause of vacuum leaks. Other common issues include cracked fittings and loose connections. Vacuum lines cost just a few bucks each, but tracing the source of the leak can be time-consuming — and expensive if you’re not performing the work yourself.

Exhaust gas recirculation valve failure

Exhaust gas recirculation valve
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The exhaust gas recirculation (EGR) system lowers the amount of nitrogen oxide that comes out of the car’s engine and helps it run more efficiently. It directs the hot exhaust gases back into the combustion chambers, which warms up the fuel and makes it easier to burn. It also reduces emissions.

The EGR valve can get clogged up or fail entirely. If you’re even slightly mechanically inclined, you can remove the valve, clean it up, and reinstall it in a relatively short amount of time. If the valve needs to be replaced, expect to pay at least $125 for a brand-new, OEM-quality unit.

Dead battery

Dead battery

The battery is as simple as it is important; without it, your car won’t start, light up the road ahead, or charge your phone. Today’s batteries last much longer than before, and they’re maintenance-free. The price of a new one depends on the type of car you drive, but plan on spending at least $100 for a quality battery.

Changing or charging a battery on your own is a relatively easy task, but keep in mind that in some late-model cars, it’s buried under countless plastic covers, and it might be a little difficult to access. Also, note that disconnecting the battery will often reset your stereo system. If you don’t have the code, ask your local dealer for it before you unbolt the positive and negative terminals. Otherwise, you’ll be driving in silence.

My check engine light is on. Now what?

Modern cars are brimming with high-tech bells and whistles, but you inexplicably still need a separate, aftermarket device to decipher why the check engine light is on. Most motorists simply take their car to the dealership. That’s the easy way out, but it’s also the most expensive route to take. If you want to skip a trip to the repair shop, spend a few dollars on a Bluetooth-compatible OBD II scanner (or a more advanced adapter) and download a compatible app like Torque from either the Google Play Store or Apple’s App Store. You can get the tool and the app for less than $15, which isn’t bad, considering many shops charge between $50 and $100 to pull codes from the ECU.

Ready? Start by finding your car’s OBD II port. It’s usually in the driver’s footwell, not far from the hood release, but sometimes it’s hidden behind the center console or in a compartment built into the floor. Plug in the scanner, launch the app on your phone, and the error codes stored in the car’s ECU will appear on the screen. Sometimes the codes are clearly explained; other times, you’ll need to do a bit of searching to figure out what something like P1301 means. Repair manuals are useful for deciphering codes, but Google usually has the answer.

Error codes are a useful diagnostic tool that can help you figure out where to start when your car needs a repair; just be careful to understand their limitations. Error codes don’t always tell the full story.

Once, when working on a relative’s car that had lost all power, the ECU codes indicated a problem with the turbo. But the issue wasn’t with the part itself — it was a cracked vacuum hose. Long story short: Having a list of codes at your disposal doesn’t always mean a straightforward solution. You may need to spend a few minutes (or, at worst, a couple of hours) poking around the engine bay to find the issue.

There are professional-grade code scanners that are more precise but also much more expensive. The scanner can only give you a code; you also need in-depth knowledge of what each code means or a useful way to find out. Alternatively, some auto parts stores will run a diagnostic test for free. However, getting a Bluetooth scanner and an app will save you time and money while making you more car-savvy. Make sure the unit you pick is compatible with your car before buying it. Some cheaper scanners won’t work with every make and model, so it pays to do your research.

Will the check engine light tell me when I should fix my car?

The check engine light provides an idea — sometimes precise, sometimes vague — of what’s wrong with a car. However, it does not replace a skilled mechanic or routine maintenance.

In other words, don’t wait until the check engine light comes on to address your car’s maintenance needs. The ECU is not going to warn you that the water pump is about to fail, that one of the ball joints is worn, or that the A/C will stop blowing cold air in mid-August. The engine light will only come on after a system error has already occurred. In some cases, the damage from that failure can be catastrophic. 

Car maintenance requires a two-pronged approach. Routine and preventative maintenance will address common issues before they fail, and timely diagnostic maintenance will take care of new or changing sounds, smells, or how it feels.

We suggest familiarizing yourself with your car’s maintenance needs. Understanding your vehicle better means you know precisely when to change the oil, when to flush the coolant, and when to replace the brake pads. If you learn your vehicle’s ins and outs, you won’t need a check engine light to tell you when something is wrong. And, you may even avoid the issues that cause the light to come on in the first place.

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