You can be driving normally, with no strange noises or obvious problems, when suddenly a small yellow engine symbol appears on the dashboard. That naturally raises the question: why does check engine light come on?
The answer to why does check engine light come on is that your vehicle’s onboard diagnostic system has detected a fault or abnormal reading in a system it monitors. Despite its name, the light does not necessarily mean the engine itself is failing. It can be triggered by the emissions system, fuel system, ignition components, sensors, exhaust components, or other engine-management systems.
Sometimes, the answer to why does check engine light come on is relatively simple, such as a loose gas cap. Still other times, the check engine light warning simply means you may have an impending misfire, catalytic converter failure, or some similar kind of event that definitely shouldn’t be ignored. The check engine warning light is one indicator that isn’t all about the light, though; rather, the more important indicator is to whether the light is steady or flashing and what the car is doing at the time the light first came on.
What the Check Engine Light Actually Means
An engine trouble light, is a warning light on a vehicle’s instrument panel that lights up and suggests further action be taken. Modern cars are equipped with on-board diagnostics which constantly monitor a vehicle’s operation and its many components. If a certain situation can not remain within tolerance limits, an error code can be stored in a cars diagnostic memory and warning light turned on.
This distinction is important, as the warning light on the dashboard doesn’t tell you which part has gone bad. The light is simply a warning that additional work should be performed.
An oxygen-sensor fault, an evaporative-emissions leak, a fuel-delivery problem or engine misfire, for example, may all bring about the same basic symbol on the dash. The code that a diagnostic scan can retrieve from an engine computer, while helpful, is not conclusive, in that it is only one piece of the diagnosis and replacing whatever part appears in the code without testing the system can prove to be a wasted expense.
Why Does Check Engine Light Come On?
There is no single culprit. If the alert lights up because of an issue with the vehicle’s air-fuel mixture, emission problem, an ignition issue or others factors related to how the engine is functioning the light can illuminated. Below are just a few more potential possibilities.
1. A Loose or Faulty Gas Cap
It’s easy to ignore a gas cap, as it doesn’t sound like something related to performance of the car’s engine, but it’s part of what’s referred to as the vehicle’s evaporative-emissions control system and it’s built to keep vapors of fuel within the tank. If the cap is loose, it breaks, or the cap is no longer capable of holding the seal the system believes it holds you might end up with the check-engine light shining for. Especially if it appears shortly after filling with gas and nothing else is wrong with how the vehicle handles, then it is appropriate to assume that the loose gas cap might be to blame.
However, you should never decide it is a gas-cap issue just for convenience.
If the light continues to shine the car requires professional diagnostics.
2. A Failing Oxygen Sensor
Oxygen sensors help gauge how much oxygen is in the exhaust and tell the computer how to fine tune air and fuel mixtures. A problem with a sensor, or circuit damage to that sensor, may trick the car’s system into interpreting abnormal values, resulting in some performance anomalies like less fuel efficiency, different-than-expected howls or, at times, nothing the driver notices. Since many issues impacting fuel systems and the vehicle emissions all mimic each other in ways and often can result in the same diagnostic codes, relying too much on those codes (and not the actual car and test result conditions) can result in parts that don’t need replace, just the correct diagnosis.
3. Spark Plug or Ignition Problems
Gasoline combustion ignites with properly timed sparks. The combustion may become unreliable due to wear to the spark plug, faulty ignition coil, fraying of a wire to the ignition coil, etc. A possible result is that the engine misfires – that is, one or more cylinders are not burning fuel and air as expected.
A misfire might also lead to: Hesitation Decelerating with an unusually rough idle Rough engine idle Losing the gas pedal Reduced acceleration Engine vibration If the light flashes – rather than remains on, constantly – in this case, do not delay getting it diagnosed because such a severe misfire may damage the vehicle’s catalytic converter.
AAA recommends finding a safe place to stop if the light is flashing and the car is losing power, vibrating, acting strangely or smells like rotten eggs, sulfur and has a rotten smell emanating out the tailpipes.
4. Catalytic Converter Problems
The catalytic converter is a section of the exhaust system designed to reduce the levels of certain pollutants found in engine exhaust. It can be triggered on by damage, inefficiency, overheating, or any of the countless other underlying engine problems you could encounter. A failing converter can also be accompanied by an engine that is running poorly or experiencing lower than optimal fuel mileage.
It’s very important to remember that a catalytic converter does not always bring about its own demise.
For example, engine misfires, if they persist, can over stress the catalytic converter. Replacing the converter alone, and failing to remedy the misfire that originally caused its damage, will result in an extremely expensive repair that doesn’t solve anything.
5. Mass Airflow Sensor Problems
This device tracks airflow and provides input to your engine’s management system to dictate the amount of fuel to injected. As deposits build up or if the sensors fail electrical, it can feed faulty air measurements to your ECU that result in problems. Symptoms often involve poor running – a rough idle or hesitant acceleration.
You can’t assume your MAF sensor needs to be replaced, based on symptoms alone.
AAA considers a broken MAF a leading cause of check engine light illumination. Many air intake and fuel related system failures look exactly like problems to the MAF. Therefore, it always needs to be tested before.
6. Problems With the Fuel or Evaporative-Emissions System
The fuel system in your vehicle does not just consist of fuel tanks and fuel injectors. In many modern vehicles today, there are fuel and emission parts that the computer tracks. Sometimes your evaporative-emissions system leaks, it may have a malfunction in a part responsible for allowing gas vapor to exit, and can lead to the computer identifying a malfunction.
Other issues may lead the vehicle running with almost no observable symptoms because it looks fine from your perspective that diagnostic work is due for.
That is among the reasons as to why it’s hard to rely on a check engine light.
7. Battery or Electrical Problems
In addition to lighting the warning lights under certain conditions of electrical dysfunction, a weak battery can contribute to the electric gremlins. According to AAA, if a battery doesn’t offer reliable electrical support, it can result in some electrical issues and this can result in you getting the dreaded check engine light. Just don’t jump to conclusions about why that light comes on.
It doesn’t mean that your low battery voltage was the blame of the check engine light. Be cautious – while low voltage could have thrown a code you might need more proof in support that. But, if your vehicle is hard starting, or another warning like on your charging system light begins to pop up it may be time to focus on the battery.
8. Fuel-Delivery Problems
The right balance of fuel, air, compression, and spark is required for the engine to function normally. Any issues affecting the fuel injection system in any way-like the injection pressure or malfunctioning components-will cause that balance to be disrupted. Possible signs of fuel system issues are sluggish response, rough idle, slower acceleration, diminished mpg, or problems initiating engine combustion.
Another common problem associated with the fuel injection system, the misfire will require even more specialized diagnosis by your mechanic.
Data analysis from injectors, ignition coils, pressure sensors will be required prior to the problem identification and repairs.
9. An Engine or Emissions-System Fault You Can’t See
Why did the warning light on my dashboard come on? Often, the vehicle may display this warning light without presenting any immediate symptom of trouble. This does not automatically indicate a more serious underlying concern for your vehicle, but it does mean your vehicle’s self-diagnostic system has detected that something in your car warrants attention from its driver.AAA advises drivers to have a steady check engine warning light diagnosed and addressed promptly, even when driving seems fine.
A Solid Check Engine Light vs. a Flashing One
The behavior of the warning light changes how urgently you should respond.
A steady check engine light
When a light shines constantly, there’s typically an underlying problem that requires attention as soon as possible. If your car is currently running properly you will likely be okay to drive a few miles. You will just want to be sure you remember the light is illuminated for a diagnosis later. As, for example, if you turn the keys, the vehicle will start and run as if there are not problems, and there is not an indicator your vehicle will overheat, but the lights is steadily lit up it is typically okay to wait to have it diagnosed.
A flashing check engine light
A flashing light is definitely worse It can be signalling an ongoing ignition issue or an engine misfire condition that may cause further damage to systems like your catalytic converter, and is not safe to ignore if you’re seeing any smoke, shaking, loss of power, unusual smells or any other erratic behavior of your vehicle. Pull over in a safe area pull over when the time is right if you suspect your vehicle has any mechanical issues take it for assistance from road-side help or a local technician Instead of driving it long distance and hope it goes well.
Can You Drive With the Check Engine Light On?
It depends on how the light behaves and what the vehicle is doing.
A steady light with no noticeable change in performance is different from a flashing light accompanied by shaking or loss of power.
If the light is steady and the vehicle appears to be operating normally, a short trip to a repair facility may be reasonable. AAA similarly notes that short-distance driving may be acceptable when nothing is noticeably wrong with the vehicle.
The situation changes when you notice:
- flashing check engine light
- significant loss of power
- severe shaking or rough running
- smoke
- unusual burning or fuel odors
- overheating
- other serious warning lights
In those circumstances, continuing to drive can turn a diagnostic problem into a more expensive mechanical failure.
What Should You Do When the Light Comes On?
Start with observations before buying a new one.
Think about when it happened. Did it occur right after refueling? Did the engine start to run roughly? Did you experience a change in acceleration? Did another light come on?
If there is a standard gas cap, try tightening it. Then, observe the light.
The most crucial step is testing.
An OBD2 scanner can read diagnostic trouble codes from your car, if it has one. AAA recommends using an OBD2 scanner because it can access the diagnostic system in your car through the diagnostic link connector. It can then read codes that can identify which system is causing the issue, usually related to the engine or emissions.
However, it might not detect the precise problem.
A diagnostic scan may identify what system is causing the issue, but not what is causing the problem. This means that a professional mechanic may have to complete more tests before they can recommend a part replacement.
Why You Shouldn’t Simply Clear the Light
Some of the drivers use a scanner to clear the code and then think that everything is OK because the light is off.
It just clears the code from the dashboard, but it doesn’t necessarily fix the problem.
Therefore, it may appear again if the problem still exists. Moreover, it can confuse the mechanic if the car hasn’t driven enough to register the problem again. This happens sometimes because people tend to clear diagnostic trouble codes without addressing underlying issues, thinking that simply removing the codes would be sufficient.
Instead, it would be better to actually fix the issue and then let the car clear the code itself.
What If the Check Engine Light Turns Off by Itself?
A check engine light can sometimes go off once the condition that caused it to illuminate no longer exists.
This doesn’t necessarily mean that nothing happened, however.
An intermittent condition can be hard to troubleshoot because the problem may not occur while the vehicle is being diagnosed. In many cases, however, there will still be enough information stored to be able to determine the cause, especially if the vehicle is equipped with a more advanced diagnostic system.
If the light is coming on and going off intermittently, it’s important to try and determine what conditions might be causing it. Is it happening when the engine is cold? When accelerating? When decelerating? When parked? When filling up the fuel tank? On the highway? In wet weather? This information can often be used to recreate the circumstances that led to the problem occurring in the first place.
Common Mistakes Drivers Make
The warning light doesn’t tell what it is that needs to be repaired. It is a common mistake to believe that just because you see a certain code it means that a certain part needs to be replaced.
A second common mistake is to ignore the message if the light is blinking but the vehicle seems to be operating normally. Some emission or engine control issues occur in a gradual manner and it is better to get them fixed before they cause other malfunctions.
The most costly mistake is to continue to drive as normal when the check engine warning light is blinking and the engine is misfiring. If the malfunction is serious enough it can be dangerous to continue driving with a blinking check engine warning light.
The last common mistake is to take the advice of a search on the internet for a diagnostic trouble code and replace parts based on this information. Different vehicles require different diagnostic procedures to determine what is wrong and each code can be caused by numerous issues.
When Should You See a Mechanic?
Any competent mechanic can tell you that a steady check engine light should be taken in for service by a qualified technician if you are unsure of the cause.
If your car or truck exhibits abnormal drivability issues, it is even more important to have it diagnosed and repaired.
Furthermore, if the light is flashing, you should consider that a more serious issue and be prepared to pull over if you smell or see something wrong, feel vibration in the steerering wheel or elsewhere, loss of power, or other abnormal condition.
The important thing is that the light is there and it means something is wrong, which is why we always say “Don’t ignore that check engine light!”
Conclusion
So, why does check engine light come on? Because the vehicle’s diagnostic system has detected a fault in a monitored engine, emissions, fuel, ignition, or related system. The cause may be relatively simple, such as a loose gas cap, or it may involve a more significant problem such as an engine misfire, sensor failure, or catalytic-converter issue.
The most basic type of differentiation is the difference between the constant light and the flashing light. If the light is constant, it means you should probably bring it in right away. If the light is flashing, especially if you have other related symptoms like shaking, loss of power, smoke, or strange odors, that means you should definitely get off the road and seek assistance.
Instead of taking a guess at what system is failing or ignoring the issue entirely, take your vehicle into the shop and have the codes read and diagnosed. After all, the vehicle is just communicating with you that there is an issue, so it is best that you take action before a minor problem becomes a major repair.
Frequently Asked Questions
1. Why does my check engine light come on but my car runs fine?
A vehicle can detect an emissions, sensor, fuel-system, or other fault before the driver notices any change in performance. A steady light with normal driving does not necessarily indicate an emergency, but it should still be diagnosed.
2. Is it safe to drive with a check engine light on?
If the light is steady and the vehicle is operating normally, a short drive to have the vehicle diagnosed may be reasonable. A flashing light or serious symptoms such as shaking, power loss, smoke, or unusual odors require much greater caution.
3. Can a loose gas cap cause the check engine light to come on?
Yes. A loose or faulty gas cap can interfere with the evaporative-emissions system and trigger the warning.
4. What Does a Flashing Check Engine Light Mean?
A flashing check engine light can indicate a serious ignition problem, commonly an active engine misfire. Continued driving can potentially cause additional damage, including damage to the catalytic converter.
5. Will the check engine light turn off by itself?
It can turn off if the vehicle no longer detects the condition that triggered it. However, an intermittent warning should not automatically be dismissed because the underlying problem may still exist.
6. Does an OBD2 scanner tell me exactly what is wrong?
Not necessarily. An OBD2 scanner retrieves diagnostic trouble codes that can point toward a particular system or condition. Further testing may be necessary to identify the actual failed component.
