Why Is My Check Engine Light On?
Few dashboard lights get a driver's attention quite like the check engine light.
You're driving normally when suddenly a small yellow or orange engine-shaped symbol appears on the dashboard.
The vehicle might still feel completely normal.
So what does it mean?
Is something seriously wrong with the engine?
Can you keep driving?
Could it be something simple?
The frustrating answer is that the check engine light can mean many different things.
A loose or faulty gas cap can potentially trigger it. So can a faulty sensor, engine misfire, ignition problem, emissions-system issue, or another problem detected by the vehicle's onboard computer.
That's why the check engine light shouldn't automatically cause panic, but it also shouldn't be ignored.
Here's what drivers should know about the check engine light, including common causes, the difference between a solid and flashing light, and what to do when it appears.
What Does the Check Engine Light Mean?
Modern vehicles contain computers that constantly monitor different systems.
One of the most important is the On-Board Diagnostic system, commonly known as OBD.
On most modern vehicles, you'll encounter OBD-II, the standardized diagnostic system used on passenger vehicles sold in the United States since the 1996 model year.
The system monitors information from sensors and components throughout the vehicle.
If the computer detects certain problems, it can store a diagnostic trouble code and illuminate the check engine light.
That light is sometimes also labeled:
Service Engine Soon
Check Engine
Or represented by an engine-shaped symbol.
The important thing to understand is that the light itself doesn't tell you exactly what's wrong.
It tells you the vehicle has detected something that needs attention.
Is a Check Engine Light Always Serious?
No.
The severity can vary significantly.
One vehicle may illuminate the light because of a relatively minor emissions-system problem.
Another may have an active engine misfire that could potentially damage other components if driving continues.
That's why you shouldn't diagnose the vehicle based solely on the presence of the light.
You need more information.
Start by asking:
Is the light solid or flashing?
Does the vehicle feel normal?
Is the engine shaking?
Has the vehicle lost power?
Are there unusual sounds?
Do you smell anything strange?
Are other warning lights illuminated?
Those details can help determine how urgently the vehicle needs attention.
Solid vs. Flashing Check Engine Light
This is one of the most important distinctions.
Solid Check Engine Light
A solid check engine light generally means the vehicle has detected a fault that should be diagnosed.
If the vehicle is otherwise running normally, you may be able to drive it cautiously while arranging service.
But don't ignore the light indefinitely.
The underlying issue could affect:
Fuel economy
Emissions
Engine performance
Other components
Long-term reliability
The sooner you understand the cause, the better.
Flashing Check Engine Light
A flashing check engine light deserves more immediate attention.
It can indicate a more serious condition, such as an active engine misfire.
A severe misfire can allow unburned fuel to enter the exhaust system, potentially overheating and damaging the catalytic converter.
If the check engine light begins flashing and the vehicle is shaking, losing power, making unusual noises, or otherwise running poorly, reduce speed and find a safe place to stop.
Continuing to drive normally may make the problem worse.
1. Loose or Faulty Gas Cap
Believe it or not, your gas cap can sometimes trigger the check engine light.
Modern fuel systems are designed to contain gasoline vapors.
The evaporative emissions system, often called the EVAP system, monitors whether those vapors are escaping.
A loose, damaged, or improperly sealing gas cap can create a leak in that system.
The vehicle's computer may detect the problem and turn on the check engine light.
If the light recently appeared after filling your tank, checking the gas cap is an easy first step.
Make sure it is installed correctly and tightened according to the vehicle manufacturer's instructions.
However, don't assume the gas cap is always the answer.
If the light remains on, the vehicle may need diagnosis.
Why Doesn't the Light Turn Off Immediately?
Let's say the gas cap was loose.
You tighten it.
The check engine light may not disappear instantly.
The vehicle's computer may need to run its emissions-system tests again and determine that the problem is no longer present.
That can take additional driving cycles.
If the problem has been corrected, the light may eventually turn off on its own.
If it stays on, further diagnosis may be necessary.
2. Oxygen Sensor Problems
Another possible cause involves an oxygen sensor, commonly called an O2 sensor.
These sensors monitor oxygen levels in the exhaust.
The vehicle's computer uses that information to help manage the engine's air-fuel mixture and monitor emissions-system performance.
A faulty oxygen sensor can potentially cause:
Check engine light
Poor fuel economy
Increased emissions
Engine-performance issues
Other diagnostic trouble codes
Because oxygen-sensor information affects engine management, ignoring a faulty sensor may create additional problems over time.
How Many Oxygen Sensors Does a Car Have?
That depends on the vehicle.
Modern vehicles may have multiple oxygen sensors positioned before and after the catalytic converter.
The number can depend on:
Engine configuration
Number of exhaust banks
Exhaust-system design
Model year
Manufacturer
That's why seeing an "oxygen sensor code" doesn't automatically tell you which component needs replacement.
The diagnostic code and additional testing matter.
3. Catalytic Converter Problems
The catalytic converter is part of the exhaust and emissions-control system.
Its job is to help convert certain harmful exhaust pollutants into less harmful substances before they leave the tailpipe.
If the vehicle's computer determines that catalytic-converter efficiency has fallen below an expected threshold, it may illuminate the check engine light.
Catalytic-converter problems can sometimes be expensive.
But there's another important point:
A catalytic-converter code doesn't necessarily mean you should immediately replace the converter without further diagnosis.
Other problems, including engine misfires or fuel-mixture issues, can potentially contribute to converter damage or efficiency problems.
Finding the underlying cause matters.
4. Spark Plug Problems
Spark plugs play a critical role in gasoline engines.
They create the spark used to ignite the air-fuel mixture inside the cylinders.
Over time, spark plugs can wear.
Depending on the engine and problem, worn or damaged spark plugs can contribute to:
Misfires
Rough idle
Poor acceleration
Reduced fuel economy
Hard starting
Check engine light
Spark-plug replacement intervals vary considerably by vehicle and plug type.
Some manufacturers specify relatively long intervals.
Others differ.
Follow the maintenance schedule for your particular vehicle rather than using one universal mileage.
5. Ignition Coil Problems
Modern gasoline engines commonly use ignition coils to provide the high voltage necessary for the spark plugs.
If an ignition coil begins failing, one or more cylinders may not fire properly.
This can cause an engine misfire.
Possible symptoms include:
Check engine light
Flashing check engine light
Shaking
Rough idle
Poor acceleration
Reduced power
Poor fuel economy
Difficulty starting
A misfire doesn't automatically mean the ignition coil is the cause.
Spark plugs, fuel injectors, compression problems, wiring, and other issues can create similar symptoms.
Diagnosis is important.
What Is an Engine Misfire?
A gasoline engine depends on each cylinder completing the combustion process correctly.
A misfire occurs when combustion doesn't happen properly in one or more cylinders.
You may feel the engine:
Shake
Hesitate
Stumble
Idle roughly
Lose power
A misfire can be intermittent or constant.
The vehicle's computer may store a diagnostic trouble code identifying which cylinder is experiencing the issue.
For example, some codes correspond to a specific cylinder while others indicate a random or multiple-cylinder misfire.
Why a Misfire Shouldn't Be Ignored
A severe misfire can do more than make the vehicle uncomfortable to drive.
Unburned fuel may enter the exhaust system.
That can cause the catalytic converter to become extremely hot.
Over time, this may damage the converter.
That's one reason a flashing check engine light deserves immediate attention.
The warning isn't simply about emissions.
Continuing to drive could potentially turn one repair into several.
6. Mass Airflow Sensor Problems
Another component that can trigger the check engine light is the mass airflow sensor, often abbreviated MAF.
The MAF sensor measures the amount of air entering the engine.
The engine computer uses that information when determining how much fuel is needed.
If the sensor provides incorrect information, you may experience:
Poor fuel economy
Rough idle
Hesitation
Reduced power
Acceleration problems
Check engine light
However, a diagnostic code involving airflow doesn't automatically prove the MAF sensor itself has failed.
Air leaks, wiring problems, contamination, and other issues can potentially create similar symptoms.
7. EVAP System Leaks
Remember the gas cap?
It's only one component of the larger evaporative emissions system.
The EVAP system includes various:
Hoses
Valves
Sensors
Vapor lines
Charcoal canister components
Other parts
Its purpose is to prevent gasoline vapors from simply escaping into the atmosphere.
A leak somewhere in this system can trigger the check engine light.
Some EVAP leaks are extremely small.
The vehicle may run perfectly normally while the light remains illuminated.
That's one reason a check engine light can appear even when you don't notice any difference from the driver's seat.
8. Fuel-System Problems
The engine needs the correct amount of fuel delivered under the right conditions.
Problems involving the fuel system can potentially cause check engine warnings.
Depending on the vehicle, this could involve:
Fuel injectors
Fuel pressure
Fuel pump operation
Fuel-pressure sensors
Fuel mixture
Other components
Symptoms might include:
Hard starting
Hesitation
Rough running
Loss of power
Poor fuel economy
Stalling
But again, these symptoms can overlap with many other problems.
A scan and proper diagnosis are much more useful than guessing.
9. Vacuum Leaks
Gasoline engines depend on controlled airflow.
If unmetered air enters through a cracked hose, gasket, intake component, or another leak, the engine computer may struggle to maintain the correct air-fuel mixture.
Possible symptoms can include:
Rough idle
High idle
Hesitation
Lean diagnostic codes
Poor performance
Check engine light
Vacuum leaks can range from relatively simple hose problems to more complicated intake-system issues.
10. Other Sensor Problems
Modern vehicles use numerous sensors.
Depending on the vehicle, these can include:
Camshaft position sensors
Crankshaft position sensors
Throttle position sensors
Engine coolant temperature sensors
Manifold pressure sensors
Knock sensors
Fuel-pressure sensors
Exhaust sensors
And many others.
If a sensor fails or begins reporting implausible information, the vehicle's computer may store a code.
But there's an important lesson here:
A diagnostic code isn't always proof that the sensor named in the code needs replacement.
Sometimes the sensor is accurately reporting a problem somewhere else.
What Is a Diagnostic Trouble Code?
When the vehicle detects certain faults, it stores a Diagnostic Trouble Code, commonly abbreviated DTC.
These codes usually contain a letter followed by four numbers.
You may have seen codes such as:
P0300
P0420
P0171
The code provides information about the system or condition the vehicle detected.
Technicians access these codes using a scan tool connected to the vehicle's diagnostic port.
What Does the "P" Mean in a Trouble Code?
Diagnostic trouble codes are divided into different categories.
A code beginning with P generally relates to the powertrain.
Other code families can relate to:
Body systems
Chassis systems
Network communication
The exact code helps narrow down where the problem exists.
But it usually isn't a complete diagnosis by itself.
Can You Read Check Engine Codes Yourself?
Yes.
Basic OBD-II code readers are widely available.
Some plug directly into the diagnostic port and display codes.
Others connect to a smartphone.
This can be useful if you want to know why the light appeared.
But be careful about jumping from:
"I have code PXXXX"
to:
"I need to replace this part."
A code tells you what condition the computer detected.
A proper diagnosis determines why that condition occurred.
Those aren't always the same thing.
Where Is the OBD-II Port?
The diagnostic connector is generally located underneath the dashboard on the driver's side.
Exact placement varies.
It may be:
Under the steering column
Near the lower dash
Behind a small cover
Closer to the center console
A technician connects a scan tool to this port to communicate with the vehicle's computer systems.
Can an Auto Parts Store Read the Code?
Many auto-parts retailers offer basic code scanning.
That can be useful for identifying which diagnostic trouble code is stored.
But remember that reading the code isn't necessarily the same as diagnosing the vehicle.
If a scan shows a code related to an oxygen sensor, for example, additional testing may still be needed before replacing anything.
Use the code as information, not automatically as a parts-shopping list.
Can You Keep Driving With the Check Engine Light On?
It depends on what's happening.
If the light is solid and the vehicle:
Runs normally
Has normal power
Isn't overheating
Isn't shaking
Has no unusual smells
Has no unusual noises
You may be able to continue driving cautiously while arranging diagnosis.
But don't ignore it for weeks or months.
If the light is flashing, or the vehicle is:
Shaking badly
Losing power
Overheating
Smoking
Making serious mechanical noises
Producing strong unusual odors
Running extremely poorly
Stop driving as soon as it's safe and seek assistance.
What If the Check Engine Light Comes On With Other Warning Lights?
Pay attention to all of them.
A check engine light accompanied by an oil-pressure warning, overheating warning, or another serious indicator may require a very different response from a check engine light by itself.
The owner's manual explains the warning lights specific to your vehicle.
Some warnings mean:
Schedule service.
Others mean:
Stop the vehicle.
Knowing the difference matters.
Check Engine Light vs. Maintenance Light
These are not necessarily the same thing.
A check engine light indicates that the vehicle's diagnostic system detected a fault.
A maintenance reminder may simply indicate that scheduled service is due.
For example, a maintenance reminder might appear because the vehicle has reached the mileage or time interval for:
Oil change
Tire rotation
Inspection
Other scheduled service
Don't assume "Service Engine Soon" and "Maintenance Required" always mean the same thing.
Check the owner's manual for your specific vehicle.
Why Did My Check Engine Light Come On After Getting Gas?
A loose or improperly installed gas cap is one possible explanation.
If your vehicle uses a traditional fuel cap, check that it's properly secured.
Some newer vehicles use capless fuel systems.
Those systems can have their own sealing components and potential faults.
If the light remains illuminated after several trips, have the diagnostic codes checked.
The timing could also be coincidental.
Just because the light appeared after getting gas doesn't guarantee the fuel cap is responsible.
Why Did My Check Engine Light Turn Off by Itself?
The vehicle's computer continuously monitors systems.
If the condition that triggered the light no longer occurs during subsequent tests, the computer may eventually turn the light off.
That can happen with intermittent problems.
For example, a loose gas cap might be tightened.
An intermittent sensor reading may return to normal.
A temporary condition may disappear.
But the diagnostic system may retain information about what happened.
If the light repeatedly comes on and goes off, have the vehicle checked.
Intermittent problems are still problems.
Can Cold Weather Cause a Check Engine Light?
Cold weather itself doesn't necessarily mean something is broken.
But colder temperatures can expose existing weaknesses or change operating conditions.
For example, winter can affect:
Battery performance
Electrical systems
Fuel vapor behavior
Engine warm-up
Sensors
Air intake
Other components
A light that repeatedly appears during cold weather still deserves diagnosis.
Don't assume it's "just because it's cold."
Can a Weak Battery Cause Warning Lights?
Electrical problems can sometimes create unusual vehicle behavior.
Modern vehicles rely heavily on stable electrical voltage.
A weak battery, charging-system problem, poor connection, or voltage issue can potentially contribute to warning lights or stored faults.
If the vehicle is also:
Cranking slowly
Having difficulty starting
Showing multiple warning messages
Experiencing electrical glitches
The battery and charging system may be worth checking.
Check Engine Light and Poor Gas Mileage
Sometimes the first thing you notice isn't the warning light.
It's how often you're visiting the gas station.
Problems involving oxygen sensors, airflow measurement, ignition, fuel mixture, and other engine-management systems can affect fuel economy.
If your check engine light is illuminated and your MPG suddenly drops, mention both symptoms when having the vehicle diagnosed.
They may be related.
Check Engine Light and Rough Idle
Does the vehicle shake while sitting at a stoplight?
Does the engine sound uneven?
Does the RPM fluctuate?
A rough idle combined with a check engine light can potentially point toward problems involving:
Misfires
Airflow
Fuel delivery
Vacuum leaks
Ignition components
Sensors
Other engine conditions
Don't immediately assume one particular component has failed.
Multiple problems can create similar symptoms.
Check Engine Light and Loss of Power
A significant loss of power deserves attention.
Some vehicles may enter a reduced-power or protective operating mode when the computer detects certain faults.
Drivers sometimes call this "limp mode," although terminology varies by manufacturer.
The vehicle may limit:
Engine power
Throttle response
Transmission operation
Maximum speed
Other functions
The purpose is often to protect the vehicle or encourage the driver to seek service.
If your vehicle suddenly has very little power, don't continue driving normally.
Why Ignoring the Check Engine Light Can Get Expensive
Sometimes drivers ignore a solid check engine light because the vehicle still seems to run normally.
That can be risky.
Imagine an engine begins misfiring because of a relatively manageable ignition problem.
If ignored, the misfire contributes to catalytic-converter damage.
Now the vehicle may need both the original repair and an expensive emissions component.
Or a fuel-mixture problem reduces fuel economy for months.
You're paying extra at the pump while the underlying problem continues.
Early diagnosis doesn't guarantee a cheap repair.
But it gives you a better chance of addressing the problem before it causes additional damage.
Should You Clear the Check Engine Light Yourself?
A code reader may allow you to erase diagnostic codes.
But clearing the code doesn't repair the problem.
It simply removes the stored information and turns off the warning until the vehicle detects the fault again.
Clearing codes can also erase useful diagnostic information that could help a technician determine what happened.
If you're trying to diagnose a problem, it's often better to record the codes and relevant data before clearing anything.
What If the Light Appears Right Before a Road Trip?
Don't ignore it because you're in a hurry.
At minimum, have the diagnostic codes checked and determine whether the vehicle is safe to drive.
A minor emissions-system issue and an active misfire are very different problems.
Finding out which one you're dealing with before driving hundreds of miles is much better than finding out on the highway.
What Should You Do When the Light Comes On?
Start with a few simple steps.
1. Look at the light.
Is it solid or flashing?
2. Pay attention to the vehicle.
Is it shaking, losing power, overheating, or behaving differently?
3. Check other warning lights.
Make sure there aren't additional serious warnings.
4. Check the gas cap.
If your vehicle has one, make sure it's properly installed.
5. Have the codes scanned.
This provides a starting point for diagnosis.
6. Don't ignore the problem.
Even if the vehicle seems normal, find out why the light appeared.
Check Engine Lights Matter When Shopping Used
If you're looking at a pre-owned vehicle and the check engine light is illuminated, don't simply assume it's "probably just a sensor."
Find out why it's on.
The same applies if the light doesn't illuminate briefly when the vehicle is first started.
Dashboard warning lights generally illuminate during startup as part of a bulb or system check.
If the check engine indicator never appears at startup, that's something worth investigating.
A scan tool can also reveal stored diagnostic information.
A Test Drive Can Tell You a Lot
When test-driving a used vehicle, don't focus only on whether you like the seats and touchscreen.
Pay attention to how the engine behaves.
Does it start easily?
Does it idle smoothly?
Does it accelerate normally?
Are there unusual vibrations?
Does the transmission behave normally?
Are warning lights illuminated?
Do any warning messages appear?
A test drive doesn't replace a mechanical inspection, but it can help identify issues worth investigating.
Don't Panic, But Don't Ignore It
That's probably the best way to think about the check engine light.
A solid light doesn't automatically mean your engine is about to fail.
Sometimes the cause can be relatively simple.
But the light exists because the vehicle detected something outside of its expected operating conditions.
Find out what it is.
If the light is flashing or the vehicle is running poorly, treat the situation more urgently.
If the light is solid and the vehicle seems normal, arrange diagnosis rather than pretending the warning isn't there.
Find Your Next Vehicle at Big City Cars
Understanding your vehicle makes ownership a lot less intimidating.
The check engine light is a perfect example.
Once you know what the warning means, the difference between a solid and flashing light, and how diagnostic codes work, that little engine-shaped symbol becomes much less mysterious.
At Big City Cars in Fort Wayne, shoppers can compare pre-owned cars, trucks, SUVs, and crossovers from a variety of manufacturers.
When you're looking at your next vehicle, pay attention to the dashboard during your test drive.
Make sure warning lights behave normally.
Listen to the engine.
Notice how the vehicle accelerates and idles.
And once you've found the right vehicle, keep paying attention to what it's telling you.
If the check engine light appears, don't immediately assume the worst.
But don't cover it with tape and hope for the best either.
Check the basics, get the code read, and figure out what your vehicle is trying to tell you.
