Every car's engine has an engine control unit (ECU) that optimizes many functions, like the air-fuel mixture, idle speed, and ignition timing. 

Using sensors located throughout the motor, the ECU performs onboard diagnostics and makes adjustments accordingly to ensure excellent vehicle performance.

Before the computer age, engines had to be tuned by hand and kept within specifications by pneumatic and mechanical control systems. 

Today, instead of needing to adjust the carburetor and listen for your engine to idle correctly, all you have to do is start your vehicle and let the car's computer do the work. 

In addition to making adjustments to enhance your motor's performance, the ECU also lets you know if there's a problem. You'll see your check engine light (CEL) illuminated when this occurs.

Often denoted by the outline of a motor or simply the words "check engine," the CEL is typically orange, amber, or yellow. A flashing CEL along with an audible alarm may indicate an urgent issue like low oil pressure that requires pulling over and turning off the engine. 

Automakers agreed to standardize their diagnostic systems under the OBD-II protocol, with an agreed-upon list of trouble codes and a universal connector to perform analysis. 

The OBD-II port on a car is usually underneath the steering column, and mechanics can even buy their own code readers to understand what might be wrong with the vehicle.

Paying attention to what’s happening under your vehicle’s hood can extend its life and also help maintain its resale value.

But when the time to replace your car does eventually come, remember buying a used one can save you thousands versus purchasing a new one. Aside from getting a good deal, where can you go for a quality vehicle? Shift's certified mechanics perform extensive 150-point inspections on every car and have complete vehicle history reports, so you know your used car feels as good as new. Beyond that, Shift's best-in-industry service contracts give you peace of mind that your vehicle will continue running strong mile after mile. With a Shift Vehicle Protection plan, not only are essential components covered, but you also receive roadside assistance if you run into trouble. Whether you happen to be in the continental U.S., Alaska, Hawaii, or Canada, help is just a toll-free phone call away, 24 hours a day, seven days a week, 365 days a year.

When you notice the check engine warning light, a mechanic can use an OBD-II tool to connect to your vehicle and read a diagnostic trouble code that signifies the issue. (Most mechanics charge $75 or so for this service.)

Or, if you're a do-it-yourselfer, at-home scan tools are available where you can complete your own diagnostic test. Online resources offer a list of OBD-II code meanings, and some apps can even tell you what the readout signifies. 

The difference between the two methods is a repair shop may be able to remedy a serious problem, while you'll need to drive or tow your car to a repair facility for the proper fix. 

You can turn off the check engine light via the vehicle's OBD-II connector, but if the issue persists, it will light up again within a short time.

Whatever the issue at hand happens to be, if it’s too complicated or costly to fix, it may be best to start your hunt for something new. But finding a used car that fits your needs can be a hassle. Local ads may not have the make or model you want, and how can you know you know you're getting a good deal? Shift's website has a large selection of fully inspected cars located across the country that are ready to go. The website is user-friendly, and you'll be able to find various makes and models to suit your needs and budget. At Shift, you'll get a fair, no-haggle price driven by powerful machine-learning algorithms and tons of data. And Shift also works with a network of trusted lenders who compete for your business, so you get the best deal on financing. Applying for financing with Shift is quick and straightforward, with no cost or obligation. Financing with Shift is so convenient that 9 out of 10 customers choose us when they need a loan.

Because your CEL might indicate anything from a loose gas cap to something more severe like engine problems or trouble with your catalytic converter, obtaining a scan as soon as possible lets you know the severity of the problem. 

You could only need to remove and tighten your gas cap for the check engine light to turn off. But in the case of something significant, like issues with the emissions system or spark plugs, it's probably a good idea to consult a mechanic.

In 2018, an automotive telematics company, CarMD, wrote a list of the 10 most common reasons for illuminated check engine lights, along with their approximate repair cost.

1. Replace ignition coil(s) and spark plugs ($391.42)

2. Replace oxygen sensors ($244.04) 

3. Replace catalytic converter(s) with a new OEM catalytic converter ($1,371.17) 

4. Inspect for loose gas cap and tighten or replace as necessary ($25.86) 

5. Replace ignition coils ($217.91)

6. Replace evaporative emissions purge control valve ($149.52)

7. Replace mass airflow sensor ($340.58)

8. Replace evaporative emissions purge solenoid ($153.70)

9. Replace fuel injectors ($449.73)

10. Replace thermostat ($244.61)

Sometimes your check engine light turns on even though there's no issue with your vehicle. It could be the result of varying humidity or something else. In that case, the light typically turns off on its own within a brief period.

When you notice your check engine light flashing, you might not feel like having it checked out. That's understandable, given that automotive repairs can be costly, and you need to save money for other obligations like fuel, insurance, and routine maintenance. 

But an illuminated CEL could mean many things, including expensive problems like malfunctioning catalytic converters or ignition coils and spark plugs needing replacement.

The safest bet is taking your car to a mechanic, who can diagnose the CEL and perform any needed repairs. But if you're a do-it-yourself type of person, purchasing an OBD-II scan tool allows you to read CEL codes independently. 

Ignoring your check engine light could prove expensive down the road, especially if an essential engine component needs attention. You'll also need the light turned off to pass inspection, so it makes sense to have it taken care of quickly. 

Once your mechanic informs you of whether or not your vehicle needs repairs and performs any required fixes, you'll be all set for the miles ahead.