The orange, engine-shaped icon that sometimes appears in your car’s instrument cluster isn’t just a nuisance — it’s a sign that something is wrong under the hood. Ignoring it could leave you stranded at the worst moment, cost you thousands of dollars, or both. The check engine light warns of issues ranging from a gas cap that’s not properly tightened to a more serious failure like a bad catalytic converter or a problem with one of the car’s oxygen sensors.
Here are the 10 most common problems that can trigger a check engine light.
Oxygen sensor failure
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. A car 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, however 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 gas cap
A loose gas 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
A 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
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
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. A 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
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 an MAF problem.
An MAF for a late-model car typically costs between $120 and $150.
Issues with an aftermarket alarm
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.
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
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 re-install 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.
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 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.
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 P1301 means.
There are professional-grade code scanners that are more precise but also much more expensive. 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.
So the check engine light tells 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 doesn’t replace a skilled mechanic.
In other words, don’t wait until the check engine light comes on to repair your car. 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 is going to stop blowing cold air in mid-August. If your car drives, sounds, or smells funny, either fix it or take it to someone who can.