Why Does Check Engine Light Come On?
Check engine light comes on sometimes out of nowhere, the vehicle seems perfectly fine. It is not clear if you should continue to drive, stop at a shop, or pull over now. Check engine light is a warning device associated with your car’s diagnostic system. It tracks and monitors: engine, emission, fuel delivery, and many other parts.
If you are wondering why does check engine light come on, the most common causes range from relatively simple problems, such as a loose or damaged gas cap, to issues involving oxygen sensors, spark plugs, ignition components, catalytic converters, fuel systems, or other engine controls.A warning light does not identify the failed component by itself.Instead, it indicates that the computer has detected a condition outside an expected range and stored diagnostic information that can be retrieved with a scan tool.
Knowing the distinction between a solid and a flashing warning light is also quite important, and the check engine light warning guidance can help drivers understand when a warning may require prompt attention. The former can alert you to a problem that might need attention relatively soon, while a flashing warning light generally means that there’s an emergency and that you need to address it immediately. Understanding what the warning implies can help you avoid a needless repair bill, but could also save a more moderate concern from becoming an costly failure.
What Does the Check Engine Light Mean?
Short answer: The check engine warning indicator comes on because the vehicle’s on-board computer has identified some system or operating parameter as being faulty or abnormal. The cause of the warning can range from simple things such as a loose fuel filler cap to potential engine miss-fires or emissions system faults.
Modern automotive systems involve many electronic control modules that are in constant communication with numerous sensors and various other automobile components. The engine systems computer continuously gathers vast amounts of information about engine speed, air fuel ratio, exhaust emissions, ignition timing, and temperature, pressure and many other engine operating parameters. When a specific issue is identified by the computer that matches certain diagnostic parameters a diagnostic trouble code is logged within the computer and the check engine warning light is activated.
Therefore, the check engine warning light in your vehicle usually just indicates a starting point to determine the problem. A diagnostic check on the vehicle systems will typically tell you what system triggered the warning and suggest or guide additional tests to diagnose the issue.
Common Reasons the Warning Light Appears
There is a multitude of causes as there are many things that the diagnostic system monitors, a few of them are cheap and easy to replace and then some needs to be diagnosed quite thoroughly.
Loose or damaged gas cap
One of the easier causes: a lose, or missing, or faulty fuel cap. Present vehicles have checks in place for the evap system (evaporative emissions control) which ensures fuel vapors do not escape into the atmosphere.
If the fuel cap does not seal properly, the system may detect a leak and trigger a warning.
If you are wondering why does check engine light come on after refueling, checking that the gas cap is properly installed is a reasonable first step. If the cap was loose, the light may not disappear immediately. The vehicle may need to complete several drive cycles before the computer determines that the problem is no longer present.
A damaged seal or cap may need replacement if the warning continues.
Failing oxygen sensor
Oxygen sensors take readings on the amount of oxygen in the exhaust gases and relay them back to the engine computer, allowing it to regulate the air-fuel ratio. An oxygen sensor is important in many aspects of fuel control and emissions; and a faulty one can throw them both off. Depending on the vehicle and type of fault, the driver may experience such issues as decreasing gas mileage, poor running, or others. Even though you got scan codes that reference oxygen sensors, it does not necessarily mean they’re the actual cause of the malfunction, and they do need to be accurately diagnosed to ensure that you replace the part needed.
Worn or damaged spark plugs
Spark plugs are used to provide the necessary spark in a gasoline engine to initiate a charge of air and fuel, and this process tends to become somewhat uncertain as the spark plug ages. Some of the possible effects of an aged spark plug is that the engine might get an engine misfire, the idle will get uneven or rough, hesitation will set in during acceleration, the engine might seem to not have as much power, or it may have an extremely tough time to get started. An out of sequence crankshaft speed variation is something that will trigger the diagnostic system as being related to combustion issues, and consequently, the check engine light.
Spark plugs on many vehicles are a service item.
Replacement intervals will range; always refer to a manufacturer’s maintenance schedule, rather than presume that each one needs the same one.
Ignition coil problems
Ignition coils are required to produce the high voltage necessary for the spark plug to create a spark, and a dead coil cylinder will produce a misfire in that cylinder. Symptoms often consist of an engine that shaking, faltering during acceleration or sometimes hesitating. In many situations, vehicle computers detect this relatively quickly by throwing a specific misfire code for that cylinder, but a specific diagnosis may be warranted as symptoms can mimic those of a number of fuel delivery problems, vacuum leaks, injectors, or combustion and compression problems
Catalytic converter problems
The catalytic converter helps reduce harmful pollutants in exhaust gases. If it becomes damaged, contaminated, restricted, or inefficient, the vehicle can trigger an emissions-related warning.
If you are wondering why does check engine light come on, one possible reason is a problem involving the catalytic converter. Catalytic converter replacement can be expensive, which makes proper diagnosis especially important. A converter may fail because of another underlying problem, such as persistent engine misfires or excessive oil consumption.
Replacing the converter without correcting the cause can lead to another failure.
Mass airflow sensor issues
The mass airflow sensor quantifies how much air is passing into the engine, and the computer used this to dictate fuel delivery. The engine cannot be properly controlled unless the computer is supplied accurate information on how much air is coming in. A dirty or failing mass airflow sensor is not the culprit as often as it is thought to be.
Such problems can cause any range from an acceleration deficiency to an unstable idle, fuel economy drops, and warning light illumination.
Cleaning the sensor is not always the solution and cleaning can sometimes damage some types of the mass airflow sensor. Also, a malfunctioning mass airflow sensor has many symptoms in common with air leaks elsewhere in the intake system, so sometimes that should be inspected too.
Vacuum or intake leaks
Unmeasured air which enters into the engine from a broken hose, gasket or intake component may distort the normally occurring air/fuel mixture ratios, small leaks may only lead to small problems, and large leaks to rough running or a “gutless’ car to drive. A lean condition diagnostic code doesn’t always indicate a need for a new oxygen sensor – the technician may need to check out the intake, vacuum lines, fuel pressure, etc.
Fuel injector problems
Fuel injectors are designed to spray very finely atomized fuel into the engine at specified volumes. Any injector that has become blocked, is leaking fuel, or has an electrical failure may upset the proper burning of the air fuel mixture. These symptoms can manifest themselves as rough running, misfiring, hesitation, bogging, and the wrong amount of fuel consumption (good or bad). It’s important to test injectors before replacement since many of the symptoms can appear the same as ignition and/or compression related problems.
Why the Color and Behavior of the Light Matter
Not every check engine warning carries the same level of urgency.
A steady check engine light
A constant illumination of the light can mean the car’s computer has detected a fault which doesn’t necessarily constitute an immediate emergency; however it should not be ignored. If you continually disregard problems they will continue to develop, potentially increasing your emissions, diminishing your economy, causing extra parts to fail etc. Get an adequate check carried out by having your car diagnosed as soon as practical.
A flashing check engine light
A flashing light deserves much more attention.
If you are wondering why does check engine light come on, it can sometimes indicate a severe engine misfire that may cause damage to the catalytic converter or other components. If the light begins flashing while the engine is running poorly, shaking, losing power, or producing unusual noises, reduce driving and address the problem promptly.
Depending on the circumstances, continuing to drive may cause expensive damage.
A flashing warning combined with severe symptoms is a reason to stop in a safe location and seek professional assistance rather than attempting to complete a normal trip.
What to Do When the Light Comes On
The best response depends on how the vehicle is behaving.
Step 1: Check for obvious changes
Pay attention to whether the vehicle is:
- Shaking or running roughly
- Losing power
- Hesitating during acceleration
- Producing unusual smoke
- Overheating
- Making unfamiliar noises
- Smelling strongly of fuel or exhaust
A warning light combined with severe symptoms deserves more immediate attention than a light appearing while the vehicle drives normally.
Step 2: Check the fuel cap
If the warning appeared shortly after refueling, make sure the cap is properly tightened and the sealing area is not visibly damaged.
Do not assume this is the cause if the light remains illuminated.
Step 3: Check other dashboard warnings
Look at the rest of the instrument panel. An oil-pressure warning, overheating warning, battery warning, or other critical indicator may change the urgency considerably.
The check engine light should not be considered in isolation.
Step 4: Have the vehicle scanned
A diagnostic scan can retrieve stored trouble codes from the vehicle’s computer, while understanding OBD-II diagnostic trouble codes can help explain how onboard diagnostic systems identify emissions-related problems.
If you are wondering why does check engine light come on, reading the diagnostic code can help identify the system involved. The code provides useful information about the system involved, but it is not always a complete diagnosis. For example, a code related to a sensor may be caused by wiring, an intake problem, an exhaust leak, or another underlying fault.
Step 5: Diagnose before replacing parts
Avoid purchasing several replacement components based solely on internet guesses or a generic code description.
A professional diagnosis can involve visual inspection, electrical testing, pressure measurements, live sensor data, smoke testing, or other procedures depending on the fault.
Can You Drive With the Check Engine Light On?
Sometimes, it is possible, but it depends on the situation.
If the light is on constantly, and the vehicle appears to be performing normally you may be able to drive the car to a mechanic’s or diagnostics specialist. However, this does not negate the need for having an inspection, you still cannot take the light as some harmless thing.
If the warning light is flashing, or the vehicle is showing serious performance problems then you may not want to continue driving for an extended distance.
If the vehicle is vibrating significantly, seems to have completely lost significant amounts of power, is running abnormally hot, is making weird noises out of the engine compartment or smells strongly of exhaust fumes or raw gas or similar, then it is highly recommended to stop immediately and call a service technician. The most appropriate decision to make is ultimately dependent on your vehicle, its warning, and what concurrently appears to be going on.
Why the Light Can Turn Off by Itself
Sometimes a warning disappears without a repair. This can happen when the computer detects that the condition has not reappeared during subsequent driving cycles.
For example, an intermittent electrical connection or temporary emissions-system condition might not occur again immediately.
A light going out does not necessarily prove that the original problem has been permanently resolved.
If you are wondering why does check engine light come on, the computer may retain diagnostic information even after the dashboard warning disappears. If the warning repeatedly comes back, the intermittent nature of the problem can actually provide useful diagnostic information and help identify the underlying issue.
Do not assume a recurring light is unimportant simply because it disappears between trips.
Common Diagnostic Mistakes to Avoid
Replacing the part named in the code immediately
The DTC is nothing more than an alert indicating a system condition recognized by the computers in your car-it doesn’t always diagnose the problem component with precision.
A sensor-related DTC might indicate the failed sensor itself, a wire, an exhaust leak, or another system fault.
Clearing the code without fixing the cause
While the light will return if the computer re-performs the appropriate test, deleting the codes will simply clear a warning, and the dashboard light will likely return again in the near future; it does nothing to solve the problem, however. Deleting the codes is also prone to erase the valuable diagnostic codes that a technician will need to see when he looks at the car.
Ignoring a steady warning indefinitely
A steady warning may not indicate an immediate emergency, but ignoring it for months can allow an underlying issue to become more expensive.
A small emissions or ignition problem can potentially contribute to additional damage if it remains unresolved.
Assuming the car is fine because it still drives
Vehicles can compensate for certain faults, allowing them to continue running while operating outside their ideal conditions.
A car that feels normal can still have an emissions, fuel, ignition, or sensor problem.
Situations That Need Extra Attention
The light appears after refueling
Start by checking the fuel cap and watching for other symptoms. If the warning remains, have the vehicle scanned for diagnostic trouble codes.
The vehicle is shaking
Shaking combined with a warning light may indicate a misfire or another significant engine problem. Avoid unnecessary driving until the cause is identified.
The vehicle loses power
Sudden power loss can create a safety issue, particularly in traffic. Pull over safely if necessary and have the vehicle inspected.
The engine overheats
An over-heating engine demands immediate attention. Because the only alert you get is the Check Engine light is not the reason that you should keep the engine working. Cool the car down.
Please do refer to the manufacturer’s direction.
Please remember that you never should take off the cooling system cap while it is hot, because the hot cool liquid under high pressure could burn you badly.
The warning returns repeatedly
A recurring warning suggests that the underlying condition has not been fully resolved or that an intermittent fault remains.
Repeatedly clearing the code without diagnosing the cause is unlikely to provide a lasting solution.
How Mechanics Diagnose the Problem
Usually, a competent diagnostic service starts with obtaining the trouble codes and the complaint/vehicle symptom.
The mechanic then may check various sensor/module real-time data. The rest of the diagnostic sequence would involve the following items based on the code(s) present: Testing electrical circuits, inspecting connectors, checking fuel pressure, testing ignition system components, checking for vacuum leaks, and checking exhaust system function.
A diagnostic sequence should never be dictated by generic “list the replacement part for the given code”, instead it should be determined by the actual vehicle being diagnosed and the nature of the failure.
This can be difficult because of the interdependent nature of automotive systems where one part has failed causing another part to produce symptoms, and the scan tool may report the latter rather than the former.
It is the difference between fixing a problem, and just replacing “part A that came up first on the scanner.”
How to Prevent Check Engine Problems
Not all problems can be avoided, but good vehicle maintenance should lessen your chances of encountering them and make regular car maintenance tips an important part of preventing avoidable problems. Refer to your manufacturer’s recommended schedule for service interval recommendations on items such as spark plugs, filters, fluids, etc. Always use the recommended fuel and fluids for the particular vehicle you are driving, keep up with fuel system and engine maintenance, and resolve unusual characteristics before they turn into major issues.
Also look out for sudden drops in gas mileage, unusual idling, poor starting response, hesitation when accelerating, or peculiar smells emanating from the exhaust-any or all could tip you off to a lurking issue.
Although not guaranteed to catch every electronic and mechanical hiccup, regular maintenance should increase the likelihood that a developing malfunction will be found before it reaches critically failed status.
When to Seek Professional Help
Contact a professional service when the cause of the issue is not apparent, the warning lamp continually appears, the vehicle experiences decreased performance.
The vehicle’s service light illuminated and flashing requires immediate service especially if engine misfires, smooth operation not achieved or the motor runs roughly.
The use of an auto professional service also comes recommended, if the proper equipment is not available or knowledge is absent for conducting tests for electrical, fuel, ignition or emission components.
Do not experiment around a hot motor, a fuel system containing pressure or containing fuel, moving belts, or a system containing a coolant supply if safety training is lacking.
A scan should be performed but interpreting the codes and symptoms, often, must be accompanied by testing that requires skill.
Final Takeaway
The check engine light can illuminate for a number of reasons; from a loose fuel cap, to any type of ignitions, fuel system, sensor, emissions hardware, or engine performance problem. While the warning itself isn’t telling you what is wrong, the way it’s acting will provide clues, as will any driving symptoms that are accompanying the warning light. Generally speaking a steadily illuminated warning light on a vehicle performing normally would warrant attention soon rather than being an emergency, while a flashing warning light or driving symptoms like a severely running engine are causes for immediate diagnosis.
It’s ok to check the obvious things; but constantly resetting the light without diagnosing the condition or buying parts based solely on a code is not efficient and is a costly and inappropriate way to address the root cause.
Instead use this warning light as a source of valuable information and retrieve the diagnostic code, diagnosis it accordingly and repair it appropriately before it becomes a bigger or more expensive problem.
Conclusion
Knowing why does check engine light come on can help you respond appropriately instead of assuming the worst. The warning is triggered when the vehicle’s onboard diagnostic system detects a problem involving the engine, emissions system, fuel system, ignition, or another monitored component.
A loose gas cap can be the solution, but also an oxygen sensor, spark plug, ignition coil, catalytic converter, fuel injector or engine misfire can have bigger things behind it. A solid light warrants an examination and a blinking light might indicate something serious such as a misfire and is more urgent.
Don’t buy replacement parts without being sure it will fix it: scan your car for diagnostic trouble codes, and find out exactly what is the problem. You may save further costs on engine performance issues, fuel economy losses, issues with the exhaust and maybe even better in terms of a further problem.
FAQs
1. Why does the check engine light come on?
The check engine light comes on when the vehicle’s onboard diagnostic system detects a fault or abnormal condition in a monitored system. Possible causes include emissions problems, ignition faults, fuel-system issues, sensor failures, and other engine-related problems.
2. Can a loose gas cap cause the check engine light?
Yes. A loose, missing, or damaged gas cap can interfere with the vehicle’s evaporative emissions system and trigger the warning. If the light appeared after refueling, make sure the cap is properly secured. The light may take several driving cycles to turn off after the issue is corrected.
3. Is it safe to drive when the check engine light is on?
It depends on whether the light is steady or flashing and how the vehicle is behaving. A steady light on a vehicle that is running normally generally calls for diagnosis soon. A flashing light, particularly when accompanied by shaking, poor acceleration, or other severe symptoms, requires much more urgent attention because it can indicate a serious misfire.
4. What does a flashing check engine light mean?
A flashing check engine light can indicate a severe engine misfire. Continuing to drive under these conditions can potentially damage emissions-control components, including the catalytic converter. Reduce driving and seek professional assistance promptly.
5. Can low-quality fuel cause the check engine light to come on?
Poor-quality or contaminated fuel can sometimes contribute to drivability or fuel-system problems, although it is not the only possible explanation for a warning light. If the light appears after refueling and the vehicle begins running poorly, have the vehicle diagnosed rather than assuming the fuel is the cause.
6. Why does the light turn off by itself?
The light can turn off when the vehicle’s computer no longer detects the condition that originally triggered it during subsequent monitoring cycles. An intermittent fault may disappear temporarily, but that does not necessarily mean the underlying problem has been permanently fixed. Stored diagnostic information may still help identify what happened.
7. Does the check engine light always mean an expensive repair?
No. Some causes are relatively simple and inexpensive, such as a loose fuel cap. Others can be considerably more expensive, including catalytic-converter or major engine problems. A diagnostic scan and proper testing are needed to determine the actual cause rather than assuming the repair cost from the warning light alone.
8. Can I reset the check engine light myself?
A scanner may be able to clear a diagnostic code, but clearing the warning does not repair the underlying problem. If the fault remains, the light may return after the vehicle completes its diagnostic checks. It is better to identify and correct the cause before clearing the code.
9. How do mechanics find the cause of the warning?
A technician typically connects a diagnostic scan tool to the vehicle’s OBD system and retrieves diagnostic trouble codes. Those codes provide information about the detected condition, after which additional inspection or testing may be needed to identify the actual failed component.
10. When should I take my car to a mechanic?
Have the vehicle checked if the light stays on, comes on every time you drive it, comes on with a drop in fuel efficiency, performance, starting, accelerating or if idling is unstable. Don’t drive normally, seek professional assistance immediately if the light flashes or if the car is running in very poor condition.
