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The check engine light is every driver's least favorite dashboard notification. But what if it comes on when the car is running just fine?

The check engine light is a tool to assist with diagnostics, and it's not infallible. That means that the check engine light can be (and often is) wrong, or at least not telling the full story. So what should you do if your check engine light comes on, but there isn't anything obviously wrong with the car?

Computer errors cause a check engine light even if there's no actual problem with the car. However, some issues like a broken O2 sensor cause few noticeable symptoms other than a check engine light, so it's best to get it checked promptly.

There are dozens of possible reasons why the check engine light comes on, and many of those reasons don't cause the car to run any differently—at first. Cars have hundreds of sensors that are there for a reason, and sometimes sensors themselves can fail and cause a check engine light. Here, we will cover the most common reasons my check engine lights come on and what to do if you notice it with or without trouble from your car.

We sourced the information in this article from automotive repair guides, OBD II system manuals, and from surveying the online car community.

Table of Contents

What is the Check Engine Light?

The check engine light is a hey display lamp on the dash of a vehicle. It is universally shaped like an engine and illuminates orange when there's a problem with the vehicle. The check engine light has been the same for decades and is an indication that the vehicle has an OBD (onboard diagnostics) computer system installed.

Virtually every car produced after the 1980s has some version of the system onboard. The accuracy and ease-of-use vary between vehicles, and years that the check engine light has remained essentially the same on all vehicles sold in America since its inception.

What Causes the Check Engine Light to Turn On?

There are many reasons why check engine lights illuminate. Typically, the basic process goes something like this. Your car has hundreds of sensors that are tied into several computer systems. Engine control computer systems run diagnostics while the engine is running and measure everything from temperature to knock.

If one or multiple sensors provide a reading that doesn't match the manufactures normal specifications, the system will throw a code. This means it will make a code available to the OBD II data port under the dash, which can be read and cross-referenced using a code reading device.

When the code is thrown, the check engine light also turns on. This alerts the driver to the possibility of a problem and also informs them that there's a code to be read through the OBD II system.

Here's an example. Picture that a car has a check engine light on, and the code reader displays 000000124. If you reference the repair manual, the code 000000124 corresponds to a manifold vacuum leak. Now you know where to look for the problem.

Most Common Check Engine Light Causes

There are hundreds of reasons why check engine lights come on. Sometimes it's a serious problem, and sometimes there are no obvious causes. That's why it's important to evaluate and check the codes every time the check engine light illuminates. More often than not, a check engine light is caused by one of these five common issues.

Bad Gas Cap

Can a gas cap really cause a check engine light? It sure can. Your car has sensors designed to check the seal of the gas cap, as it must be able to contain the fumes of evaporated gas. If these sensors detect a bad seal, they'll throw a code and activate the check engine light. A bad gas cap is one of the cheapest and easiest causes of a check engine light, as a replacement rarely costs more than $15.

Bad O2 Sensor

An O2 sensor is a vital piece of your car's computer-controlled emissions system. This sensor detects the concentration of oxygen in your system and sends critical information to the vehicle's computers. A bad O2 sensor may not show obvious signs, but it will cause a check engine light and other subtle issues.

A bad O2 sensor can cause your car to idle improperly, and it can also reduce your gas mileage. A car with a bad O2 sensor will fail an emissions inspection in most states, so it's important to get this component replaced as soon as possible.

Bad Catalytic Converter

Catalytic converter failures are extremely common, especially as vehicles age. If your car is equipped with an OBD II system, it is almost certainly also equipped with a catalytic converter. When this device inevitably fails, it may trip sensors in the exhaust system that cause the check engine light to illuminate.

Other Emissions Components

The emissions control system on your car is complex and includes many components. Examples of these include the air pump, sensors, the catalytic converter, the pre-cat, the EGR system, and all the associated computers. More often than not, a check engine light is caused by a failure of one of these components or a random error in the computer system.

Transmission Problems

A slightly less common cause of a check engine light is a problem with the transmission. Modern cars have computerized transmissions, which are often integrated with engine control computers for efficiency. Sometimes, a car doesn't have a separate light for the transmission. Instead, if sensors detect a problem, it'll illuminate the check engine light and throw a transmission-specific code.

Is the Check Engine Light an Emergency?

So, if the check engine light is on, does that always mean there's something seriously wrong with your car? Does it mean that there's anything wrong at all? The answer is no to both counts, as the majority of check engine lights are caused by minor issues. Sometimes, the check engine light comes on without an obvious reason.

That said, there are a handful of very serious issues that manifest themselves first with a check engine light. It's essential to get your check engine light diagnosed fast to make sure there isn't a serious problem with the car.

If you're worried, you can look for other signs of trouble. Serious car problems are usually with multiple symptoms. For example, if your check engine light comes on and you hear a loud ticking or knocking, there's a good chance you have a serious problem with your engine.

Additionally, if the check engine light accompanies low oil pressure or high engine temperature, you should stop driving or bring the car to a mechanic promptly. This also applies if you notice a change in how your car drives, such as slow acceleration, rough idle, or hesitation.

Diagnosing a Check Engine Light

Diagnosing a check engine light seems easy, but there are multiple steps to the process. First, mechanics look for obvious signs of trouble, such as the ones we've covered above. Next, they connect a code reader to the vehicle's OBD data port which is usually located underneath the driver-side dash.

When the code reader displays a code, mechanics cross-reference it with a chart that applies to your specific vehicle. Some codes are universal, but others are specific to makes and models. You can also use guides on the Internet to diagnose your check engine light code.

If the code is something serious, the next step is to determine if there's a problem with the part or simply with the sensor. Sensors fail just as often as mechanical parts themselves, and computers are often the source of a check engine light.

If there is a problem with the car, the next step is to repair it. This process depends on the cause of the problem.

Check Engine Light False Alarms

Sometimes you’ll notice a check engine light even though the car runs fine. There are a couple of reasons why this may be. If there is a problem with the car, it's probably the O2 sensor. This is an extremely common problem with modern cars and a relatively easy one to fix. The gas cap could also be the culprit, as gas caps have a limited service life but are rarely replaced by owners or dealers.

But sometimes, your car could be running fine because it is completely fine. This is a check engine light false alarm. If the code reader displays a nonsense code or doesn't display a code at all, there's a good chance that a computer error is responsible for triggering the check engine light.

What's the problem is resolved, you can use a code reader to clear the code from the system. If your car continues to run fine and the check engine light does not come back on within a few days, there's a good chance you solved the problem or that there never was one.

What to Do if a Check Engine Light Comes On

So, what's the best thing to do if your check engine light comes on? The answer depends on your comfort level and experience working with cars. If you're comfortable diving into the issue yourself, you can pick up a code reader from an auto parts store for about $30. All you have to do is plug it in and read the code, then reference it with a guide for your make and model.

If you own a code reader, you can clear the code yourself and turn the check engine light off. This is only recommended if you're confident that there's no issue with the vehicle itself and that the code returned didn't specify the concerning part. But if your car throws a serious code or the check engine light comes on again after clearing the code, it's time to bring it into the shop.

If you're in a hurry, you can stop by any auto parts store. Many of these places will read the code for free, which is convenient if you're not particularly concerned about the engine. Any competent mechanic will be able to read and clear a check engine light code that may be a false alarm and require no actual attention.

Check Engine Light On But Car Runs Fine (Causes & Solutions)

About The Author

Charles Redding

Charles Redding

I've spent many years selling cars, working with auto detailers, mechanics, dealership service teams, quoting and researching car insurance, modding my own cars, and much more.

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