How To Diagnose Your Check Engine Light

Whether you own an older or newer car, seeing your check engine light illuminate on your dashboard can be stress-inducing. Not only are you concerned about your vehicle, but you may also be worried about potential repairs and how much they might cost you.

Thankfully, most modern vehicles come with onboard computer systems that make it easy to diagnose the most probable causes of engine problems and other issues. Generally, it's best to leave this diagnostic work up to an experienced mechanic at your local auto repair shop. However, it is possible to diagnose your own check engine light if you need to.

What Is a Check Engine Light?

Check engine lights on cars are often very misunderstood, so let's start with a little overview of what these lights indicate and how a car's diagnostic system works.

A check engine light on a car usually illuminates on the dashboard near the speedometer or odometer. It may be yellow, orange, or even red in color. When illuminated, the light itself may display an engine icon or even say something along the lines of, "check engine." You can find out exactly what your car's check engine indicator looks like by referring to your owner's manual.

Check engine lights with onboard diagnostics have been standard on every vehicle in the United States since 1996. Also around this time, a universal diagnostic system with a standardized list of trouble codes called OBD II or OBD 2 was established.

When a check engine light comes on, it simply means that a sensor somewhere in the vehicle has identified that something isn't operating within its normal parameters. On some vehicles, a more serious problem will result in the check engine light blinking on and off to attract the driver's attention, though this can vary between makes and models. For example, some models may utilize different colored check engine lights to indicate the severity of the issue: a yellow light may indicate a less serious problem, whereas a red check engine light may mean the problem is more severe.

How To Check Engine Light Codes Without a Scanner

The standard method for determining the cause of a check engine light is to use an OBD II (or OBD 2) scanner or code reader. These handheld scanning tools can plug directly into a vehicle's computer port. Generally, the OBD II Port location is on the driver's side of the car, but different make/models have different locations. This video offers a good guide on how to locate your port. If you can’t find your port easily, you can check your owner’s manual or search online for the location.

From there, the OBD II codes related to the issue will display on the scanner's screen. Each fault code corresponds to a different issue, allowing mechanics to troubleshoot the problem. One or multiple diagnostic trouble codes can come up, depending on the extent of the problem.

Using an OBD II scanner is the easiest and fastest way to figure out the most likely causes of your check engine warning light. Any dealership or automotive mechanic will have one of these scanners available to run a diagnostic test. Your local auto parts store may also have access to a scanner. But what if you don't have time to get to a dealership and need to check your engine light codes without a scanner?

Though not ideal, it is possible to check engine light codes on most makes/models without a scanner. There are two common methods: the "key in the ignition" technique, and the "check the odometer'' technique. Keep in mind that these methods are only effective for cars with OBD II computer systems, so they might not work on vehicles with model years earlier than 1996.

1) The "Key in the Ignition" Technique

With this method, you'll use your ignition key to illuminate OBD II codes on your car's dashboard. Insert the key into the ignition and turn it from the OFF to the ON position multiple times without cranking the engine. Generally, you'll need to do this three to five times or until you see all the lights illuminate on your dashboard.

Typically, the lights on the dashboard will then stay illuminated for a second or two before they shut off. The exception is the check engine light, which will stay on. When this happens, take a look at your odometer. This is where you'll see your OBD II code(s) displayed. Take the time to write them down as they flash. From there, you can check your car's manual or do a quick internet search to determine what each code means.

2) The "Check the Odometer" Technique

Another method for determining what a check engine light means without a code reader involves using the trip or reset buttons on your car's odometer. Start by pressing your odometer's trip or reset buttons while simultaneously turning your ignition key to ON. Once the ignition is turned to ON, let go of the reset or trip buttons and look at the odometer. You should see any relevant OBD II code(s) flash across your odometer. Write down each code as it appears.

From there, you can determine what each code means by looking at your owner's manual or searching for each exact code online. The RepairPal website has an OBD II trouble code index that makes it easy for you to enter your trouble code and explore potential fixes.

If this method doesn't work the first time, try again. You may need to play around with the number of times you need to flip the ignition from OFF to ON.

Why Is My Check Engine Light On?

The only way to determine why your check engine light is on is to obtain the diagnostic codes from your car's computer. You can do this using an OBD II scanner or either of the above methods. There are many common reasons for a check engine light to illuminate, ranging from relatively minor to serious and urgent. Some common causes of a check engine light turning on include:

Loose gas cap
Transmission issues
Failed oxygen sensors
Bad catalytic converter
Powertrain control module failure
Failed mass airflow sensor
Bad spark plugs or loose spark plug wires
Other engine problems

In general, if your check engine light comes on while you're driving, there are a few steps you should follow. Start by noticing if there are any obvious signs of urgent problems with your vehicle, such as engine misfire, vapors coming from your hood, or sluggish performance. If not, and if the check engine light is steady and solid, the issue is most likely not an urgent one. For example, you might have a bad O2 sensor or another failed sensor.

If you notice anything "off" with your vehicle's performance or if the check engine light flashes, this could indicate a more serious issue. In this case, you'll want to slow down and pull over (when safe to do so). From there, if you have roadside assistance (through your insurance or a service like AAA), they may be able to service your vehicle and resolve your issue. If the problem is more significant, you may need to have your car towed to a local dealership or mechanic to avoid any further damage.

How To Turn off the Check Engine Light

Once you determine the cause of your check engine light and take care of any necessary repairs, the light should shut off on its own. This is because your car's diagnostic system utilizes a series of sensors that constantly monitor your car's performance. Once the problem causing the fault code is resolved, the sensors should determine that everything is running normally and shut off the check engine light automatically. Sometimes, you may need to drive around for a few minutes before the light will shut off.

In some cases, however, this may not happen. If you need to shut your check engine light off manually, there are a few ways to do this:

Turn the car on and off three (or more) times
Disconnect and reconnect the battery
Use an OBD II reader

However, you may also want to check your owner's manual for a step-by-step guide to clearing the check engine light for your particular vehicle.

It's important to note that you should only clear your error codes manually if you've addressed the root cause. Otherwise, your check engine light will just come back on as soon as your vehicle's onboard diagnostics system detects the same issues.

Many problems that cause a check engine light to illuminate can lead to bigger issues if left unrepaired, such as engine damage or damage to other expensive parts. In the meantime, your vehicle may also suffer from increased emissions and poor fuel economy, which could cause your car to fail an emissions test.

Take Action When Your Engine Light Is on and Get Insurance With Clearcover

The check engine light is the last light you want to see on your dashboard while you're driving. However, the good news is that many causes of check engine lights are relatively minor and inexpensive to repair. Still, it's important to take care of even the most minor vehicle problems before they become more serious (and potentially dangerous). From there, you can keep your car in good repair and stay safe on the road.

You can also enjoy extra peace of mind behind the wheel with the right auto insurance coverage. At Clearcover, we offer smart, straightforward insurance and puts our customers’ needs above all else. No jargon or confusing terms—we clear up all your questions about claims and coverage with reliable, honest support. Get started with a free, no-obligation quote to find a perfect-fit policy you’ll never second-guess.