Whether it’s an improperly closed door or the dreaded check engine alert, dashboard warning lights are how your car communicates with you when something goes wrong.
The check engine light is the one car owners hate most. Usually, this warning is in the shape of a silhouetted engine, though sometimes it’s an even harder-to-miss all-caps “CHECK ENGINE” message.
While it seems like the scariest, this light is one the most ambiguous because it relates to anything having to do with the car’s motor and emissions system. The potential issue could be as minor as a loose wire, an ill-fitting gas cap, or a worn solenoid. Or it could mean something far more serious is wrong in the heart of the engine itself.
If the check engine light illuminates while you’re driving and everything seems fine with the car, don’t panic, but don’t ignore it, either.
What Does the Check Engine Light Mean?
A check engine light or malfunction indicator lamp (MIL) is a tell-tale that a computerized engine-management system used to indicate a malfunction. Found on the instrument panel of most automobiles, it usually bears the legend engine, check engine, service engine soon, maintenance required, emiss maint, or a pictogram of an engine—and when illuminated, it is typically an amber or red color.
The light generally has two stages: steady (indicating a minor fault) and flashing (indicating a severe fault). When the MIL is lit, the engine control unit stores a fault code related to the malfunction, which can be retrieved—although in many models this requires the use of a scan tool.
This warning light can indicate almost anything from a loose gas cap to a serious knock in the engine.
The check engine light is part of your car’s so-called onboard diagnostics system. Since 1996, every new car and light-duty truck sold in the U.S. has been legally required to have an On-Board Diagnostic system (OBD).
This is a computer that monitors emissions levels and other vital engine components. With an OBD scan, sourcing a problem is easier—something as simple as tightening the gas cap might solve the issue. This also prevents an emissions-spewing vehicle from operating without the driver knowing there’s a problem lurking under the hood.
What does a check engine light look like?
Why is My Check Engine Light Flashing?
A flashing check engine light indicates a much more serious issue. Most often it means that the engine is misfiring, and unburned fuel is getting into the exhaust system.
This raises the temperature of the catalytic converter and can cause critical damage. If the check engine light starts flashing you should immediately pull over, and shut off the engine.
A blinking light, or in some cars a red light instead of a yellow or orange light, indicates a problem that needs immediate attention. Either way, you should have the vehicle checked by a mechanic.
In late-model cars, a blinking light usually indicates an engine misfire so severe that unburned fuel is being dumped into the exhaust system, where it can quickly damage the catalytic converter, leading to an expensive repair. If that happens, you should reduce power and have the car or truck looked at as soon as possible.
If the light is steady, the problem is not an emergency, but you should schedule an appointment as soon as possible.
What to Do When the Check Engine Light Comes On?
Once you notice your check engine light has come on, pay attention to how the car is driving. Does something feel off? If so, reroute yourself to the nearest mechanic to get the car checked out. If the car seems to be running fine, you can check a few things on your own before heading to the mechanic.
There are a couple of simple solutions if your check engine light came on after getting gas. A loose gas cap is one of the most common reasons for the check engine light to go on. Check that your gas cap is screwed on securely and that it’s in good condition.
If you have a tank of bad gas in the car, your check engine light may come on. For example, if your vehicle is designed to run on premium gas (octane grade 91 to 94), the engine may struggle to run on regular gas (octane level 87).
You can safely solve this problem by having a professional drain your gas tank. If you must drive until the tank is empty again, this may not cause severe damage, as long your car doesn’t show signs of overheating or poor engine performance.
Do you suspect something else caused your check engine light to turn on? The most foolproof method to check the engine light is to read the diagnostic code in the car’s internal computer.
If you have experience working with cars, you can do this at home with an OBD-II scanner. Otherwise, head to your local service station where a mechanic can read the diagnostic code and determine the correct fix.
If you have a flashing check engine light, try to find a safe place to stop. Don’t rush, as you want to avoid adding stress to the engine. Once you’ve parked your vehicle, turn off the engine. Immediately schedule a check engine light service, or better yet, get a mobile mechanic to come to your aid.
Knowing what to do when the check engine service light comes on can save you from costly repairs.
But what causes an illuminated engine light in the first place?
Why Is My Check Engine Light On?
The reasons for a check engine light turning on can range from something minor like a loose gas cap to something more serious like a faulty catalytic converter. It could even be a sign of internal engine failure. Ignoring the problem can turn a potentially quick fix into a costly and time-consuming repair.
Common minor and major reasons for a check engine light to turn on include:
- Loose gas cap
- An internal problem in the engine
- An issue with or even theft of the catalytic converter
- Faulty spark plug, spark plug wires, or ignition coils.
- Malfunctioning oxygen sensor
- Damaged mass airflow sensor
- A bad tank of gas
1. Catalytic Converter Failure
Hopefully, your check engine warning isn’t coming on because your catalytic converter is going out. The only reason we say this is that it can be expensive to replace the catalytic converter and your vehicle cannot run without it.
The catalytic converter is part of your vehicle’s exhaust system and if you notice a rotten-egg smell along with the check engine warning, this is probably the problem.
2. Loose Gas Cap
The lines and valves in your gas tank system recirculate gasoline vapors and keep them from escaping. If the gas cap is loose, this little detail alone will quickly trigger the “check engine” light. One of the more easily addressed issues associated with the CEL, a faulty or missing gas cap may cause you lose fuel due to evaporation or mean your fuel system fails to circulate properly.
3. Faulty Modules and Sensors
Your engine control unit (ECU) utilizes multiple sensors. Issues with sensors, like a loose oxygen sensor wiring, a clogged-up mass airflow sensor, or a faulty oxygen sensor, can cause the check engine light to come on.
For example, the oxygen sensor measures the amount of unburned oxygen in your exhaust and informs your ECU, which uses this data to adjust the air-fuel ratio. A faulty O2 sensor may cause your engine to burn more fuel than required, resulting in poor fuel economy.
4. Other Emissions Problems
The emissions/exhaust system is the most likely culprit for many check engine lights being illuminated. There could be an exhaust leak or a problem with the catalytic converter. The mass airflow sensor (MAF) is another sensor that is easily tripped or can fail. It’s usually a good idea to get your exhaust system inspected if your check engine light is on.
5. Spark Plugs
Finally, worn spark plugs or plug wires will result in a check engine warning. Just about anything wrong with your vehicle’s ignition system will likely cause your check engine light to come on. If you haven’t had your plugs changed and your vehicle has over 100,000 miles on it, it’s time for new plugs and possibly new wires. Stalls are another indicator of spark plug trouble.
6. Wiring Issues
From the battery to the spark plug wires and the ignition system in between, there is a lot of wiring that is vital to your vehicle firing up and staying running right. Any loose connection or damaged wire can cause trouble and may illuminate the check engine light.
7. Transmission
Your car’s transmission manipulates engine power and transfers it to the drive wheels. Since the transmission and engine work together closely, a transmission problem (like a slipping transmission) can cause poor fuel efficiency.
Therefore, if the control module detects a problem with the transmission, it’ll activate the service engine light.
8. Overheating
If the engine coolant hasn’t been changed in a while, it can degrade the engine thermostat and lead to overheating. In such cases, your check engine light will turn on, and the temperature gauge on your dashboard will rise.
When this happens, stop driving immediately. The error code P0217 may accompany the service light.
9. Fuel System
If your engine isn’t getting the proper amount of fuel or the fuel itself is contaminated because the filter needs to be replaced, the check engine light may come on to let you know the fuel system is affecting the vehicle’s engine performance.
10. Engine Oil
You may be low on oil or the oil itself may be old and contaminated. This will lead to engine problems and loss of performance that may trip the check engine light. You might just be overdue for an oil change or there may be a leak somewhere in the fluid system.
Other Reasons Why Your Check Engine Light Might Be On
The MIL is also illuminated prior to starting the engine, along with other tell-tales on the dashboard, to demonstrate that the lamp is working and not burned out. The lamp will turn off once the car is started if no monitored faults exist.
The MIL will also illuminate if the engine has been cranked but failed to start after returning the ignition to “on”. In most modern vehicles this is usually due to not giving the engine enough cranking time to start.
In pre-fuel injection years many cranking attempts were sometimes required to start the car. If the engine fails to start on the second or third attempt, then an actual visual check of the engine could be required.
If the engine suddenly stalls or is triggered by an overload, such as on a manual transmission car, the MIL will also illuminate until the engine is started again.
How To Reset Check Engine Light?
There are 4 well-known methods for resetting your check engine light at Your Own Risk.
- Turning your car on and off again 3 times.
- Disconnecting and reconnecting the battery.
- Driving and letting it clear on its own.
- Resetting the check engine light with an OBD2 scanner.
1. Turning Your Car on And Off Again 3 Times.
The safest way to perform a Check Engine Light reset without a scanner is by turning the vehicle on and off three times. Just start the engine, wait for a second or two, and then turn it off—three times.
Restarting the car 3 times forces the car’s computer to re-evaluate the issue and determine whether the problem persists. The Check Engine Light should disappear if the issue has been resolved. If the light is still on after, the issue you attempted to fix may not have been resolved, you may have made the wrong diagnosis, or it may be a more serious issue requiring a scanner or battery disconnect to reset.
2. Disconnecting and Reconnecting the Battery.
When you’re ready to reset your engine light, follow these simple steps:
- Turn off your engine and put on safety glasses and gloves.
- Locate the negative terminal on your car battery. It will have a black cap and a negative (–) symbol.
- Using a wrench, loosen the nut on the negative battery terminal and pull the negative connector from the battery. Keep it disconnected for 15 Minutes.
- Reconnect the negative cable and re-tighten it using your wrench.
Disconnecting your battery will likely also reset your infotainment system settings, so you might need to reset the time on your dash and your favorite radio channels.
3. Driving and letting it clear on its own.
Your vehicle’s computer operates in cycles, so if you just had it serviced, repaired, or worked on, your check engine light may not have had time to take a new reading. In other words, the light’s on for the original reason the light came on—even if you resolved the problem. The light should probably go off after a few days of normal use.
If you don’t drive all that often, now is the perfect time to take that shopping trip where you need to go way out of town!
The mechanic normally resets the computer codes before returning your car to you, but they may have simply forgotten this time.
This is most common on older vehicles with more primitive computer systems.
4. Use an OBD Code Reader
You will need a wrench and an OBD (onboard diagnostic) reader. These inexpensive readers analyze and clear engine codes. To reset the light using an OBD scanner:
- Plug the reader into the OBD port of your vehicle. This port is usually located on the underside of the dash area just above the foot well. On some vehicles, you’ll have to remove the door on your fuse box to access this port.
- Once you have connected the reader, press the “enter” button to turn it on.
- The device will scan your engine for any error codes. Once done analyzing, the reader will display any error codes it finds. Make note of the codes (if any) if you wish to cross-check them in the code reader’s manual or online.
- Now you can press the “scroll” button to find the option to erase the code. Press enters while on this option to erase the fault code and reset the check engine light.
How Long Can You Drive with Check Engine Light On?
You should only drive with the Check Engine light on for 50 to 100 miles before at least scanning the computer for error codes.
If the issue is with one of your vehicle’s sensors, driving around for a while can allow it time to recalibrate or reset, which will shut the Check Engine light off.
If the light persists after driving for a while, scan the engine computer—once the problem is identified, it is up to you to decide whether to fix the issue immediately or continue driving with the light illuminated.
Ultimately, the length of time you can drive with the Check Engine light on comes down to what your attitude towards fixing the issue is.
Note: If your Check Engine light turns on, have the engine computer scanned for error codes as soon as possible to identify the issue. Once the problem is diagnosed, it is up to you how long you want to continue driving with the light on.
What You Can Do About a Check Engine Light?
Now that you’ve decided not to ignore your check engine light, you can bring your car in to find out the case. Or you can also figure out what’s going on yourself.
Fix recommends you buy a pocket scan tool—they work in cars made in 1996 and later. Called OBD-II or “on-board diagnostics second generation,” the tool will automatically read diagnostic trouble codes. You can get them for about $50 online or in auto parts stores.
“You can plug the scan tool into the OBD II port, a 16-pin connector under the dash,” Fix says. “If you get a code, write it down.” Then check your owner’s manual to see what needs to be repaired. But before you bring your car in for service.