Why Does My Check Engine Light Come On and Off
What you will learn :
Understanding the Check Engine Light
The check engine light is the main notification of the vehicle emissions system. It is a light in the shape of an engine silhouette on your vehicle dashboard (some lights may say “check engine”). Your engine is a mechanical air pump that produces emissions. These emissions have to be kept within a certain parameter. When the check engine light is on these parameters have been breached or malfunctioned. Much like a pump your engine is sucking in air and pumping out exhausts from combustion. It uses a combination of air, fuel, and electricity to operate along with a host of other mechanical components, electrical components, and liquids. Because there are so many variables in an engine the check engine light offers the best route of investigation pointing to a specific area. This makes diagnostics easier than troubleshooting the whole system.
The check engine light is tied to a code. It can be tied to many different codes, all with a specific meaning. This gives insight into the actual issue concerning the check engine light. Once the light is on, a diagnostics tool can be plugged into the car computer, and the code will be read and deciphered from that point.
Perspective Box: The mindset behind the check engine light is to feed you and mechanics information about what the engine is doing. It is a monitoring system for how the engine is currently performing. This performance can either be good, bad or in between, which indicates major or minor issues. Either way, it lets you know something is happening. That being said once you see a check engine light, there is no need to be worried just yet unless it is either flashing or red.
Indicator Differences
They’re one of three possible check engine lights that can occur within your vehicle. You can usually refer to your vehicle owner’s manual for your specific model’s meanings and differences. Here are the three possible lights and their meaning that coincide.
Steady check engine light (Usually amber or yellow) – A steady check engine light means that the light is on and stays on. It is not flashing or pulsating, it’s constant. When this type of light occurs, the sensors pick up a parameter that has been breached or malfunctioned. In other words, something needs to be fixed. This doesn’t mean that the car is not drivable however, Caution is to be taken.
Flashing/Blinking check engine light (Usually amber or yellow) – A flashing/blinking check engine light is when the check engine lights come on and off consistently, similar to when your indicator turn signals. This type is light is meant to indicate something more serious has occurred. Flashing check engine lights are to be taken seriously. One must pull over when it is safe to do so. This helps to protect the engine rather than risking further damage.
Red check engine light – This light illuminates a steady red light in the silhouette of an engine. It means the same as a blinking check engine light. This means it indicates a serious problem and you should safely stop the engine from running. Some vehicles depending on the make and model illuminate a red check engine light.
Common Reasons for the Check Engine Light Turning On and Off
There are many reasons for a check engine light, but over the years, especially recently, here are some common reasons you might see your check engine light come on and off:
Loose or Faulty Gas Cap – your gas cap on your car performs a seal on the hand tank preventing either gas vapors from exiting or unwanted air to enter. If this part is loose or faulty this can cause a problem in the air-fuel mixture for the engine combustion. Once these events occur the check engine light can come on. This doesn’t mean you can’t drive the car for this type of issue but the check engine light is giving you a heads-up to get the emissions parameters back into alignment.
Oxygen sensor (O2 sensor) issues – your engine runs on an air-fuel mixture for combustion and this mixture has a specific ratio for your engine’s expected performance. This ratio is one of the emission parameters that manufacturers set for your engine performance. Your engine has a computer that uses a sensor to measure this ratio called an Oxygen/O2 sensor. This sensor monitors the exhaust gases to gauge the amount of oxygen within the exhaust. The ratio is derived based on the oxygen levels in the exhaust and is controlled based on the reading. This is monitored to keep the ratio consistent. If the sensor is faulty this can trigger a steady check engine light. In this case, the vehicle can still run with higher emissions. However, if the issue persists it can get worse and trigger a blinking/flashing check engine light.
Mass Airflow Sensor Problems – The Mass Airflow Sensor provides data on how much air is flowing into your engine’s cylinder. With this data, the car’s ECU (computer) can properly distribute fuel into the mix for the desired ratio for combustion. If this part fails, it can potentially send the wrong information to the car’s ECU which can cause a domino effect of issues. This will result in a steady check engine light and then a blinking one if it gets worse.
Catalytic Converter Troubles – Catalytic converters are a part of your car’s exhaust system. It takes care of the rest of the un-burnt fuel from engine combustions thus lowering your emissions. The 02 sensors on your car give an accurate reading of the condition of the catalytic converters. If the readings are off the catalytic converters could be broken or clogged which will trigger a steady check engine light. This type of issue can cause you to fail your emissions test.
Less Common Causes
Faulty Spark Plugs or Ignition Coils – Your spark plugs and ignition coils are an integral part of your engine’s combustion. They take electricity from the battery to ignite the compressed fuel and air mixture providing the power stroke in the engine’s combustion cycle. You can call this the source of your engine’s mechanical power. This stroke is perfectly timed and in a specific sequence with the other cylinders. If the spark plug or ignition coil becomes faulty, it can result in a cylinder misfire dropping engine power and wasting fuel. Such events can lead to higher emissions. This type of issue can cause serious damage to parts like your catalytic converter, hence why it triggers a flashing check engine light.
Vehicles Computer or Electrical System – Your Vehicle has a computer Called an Electrical Control Unit (ECU) it is connected to the rest of the vehicle via electrical wires and is in control of much of the engine dynamics. There are two ports on the ECU, input and output. Much of the engine’s activities come through the input giving the ECU vital status information to adjust accordingly. The information flow between the engine and the ECU is connected with electrical wire which can sometimes get corroded or damaged. This can trigger a check engine light because the information flow is now faulty. Electrical damage or bad built quality can cause the check engine light to come intermittently as well.
Vacuum Leaks – Your engine uses an air-fuel ratio of 14.7:1 for combustion. One key thing in keeping your engine running well is how well the seal is kept. Around the engine, there are a variation of gaskets and rubber seals to keep things in order. Particularly in the area where the air gets sucked into the engine, there are a bunch of seals and hoses creating a channel to direct air into the engine. If any of these hoses or seals are compromised it can lead to extra air getting into the system which will cause the car’s ECU to add more fuel to the mix. This can throw off the desired emission thus triggering a flashing check engine light.
Diagnosing the Issue
Initial Steps – For steady or flashing check engine lights, it is good to go through the basic checks that can cause the check engine light to come on in the first place. Especially for steady lights. This can potentially save you a lot of time or money.
1. Check the most common issue, “Loose or faulty gas cap”. Loose or faulty gas caps can be very easy to overlook so be sure the double-check. Your check engine light could be a half turn away from coming off.
2. Turn the car on and off at least three times. The electrical system in a vehicle can be known to cause check engine lights intermittently. This issue can fix itself once the computer gets a chance to reset.
3. Use an OBD-2 Scanner – Your car’s computer can be read with an OBD-2 scanner. All modern cars come with an OBD-2 port that the scanners connect to. The scanner will give you a reading of different information on the current status of your car. You can pick up an OBD-2 scanner anywhere from $15 – $50 with varying capabilities. On the other hand, you can get a free reading from any AutoZone. The point of using the OBD-2 scanner for a check engine light is to read the check engine light codes to make it easier to diagnose and fix issues. When a check engine light comes on, there are digital codes that have different meanings depending on the issue. Here is a table of some most common codes you might see on an OBD-2 scanner, with their respective meanings.
| OBD-2 Scanner Common Codes | |
| P0300 | This means there is a cylinder misfire. |
| P0301 – P0308 | These codes refer to the direct cylinder with the misfire. Depending on how many cylinders your engine has, the number at the end of the code refers to the exact cylinder misfire. For eg. a P0302 means a misfire on cylinder number 2. |
| P0420/P0430 | You may see these codes depending on if you have a 4, 6, or 8-cylinder engine. However, they both mean that there is an issue with your catalytic converter. |
| P0141 | This means there is an issue with your O2 sensor, particularly the one after the catalytic converter. |
| P0455 | This most likely means that you either have a loose or faulty gas cap. The system senses that there is a large enough EVAP leak in the gas system that probably points to the gas cap not having a proper seal. |
| P0171/P0174 | This code points to either too much air in the air-fuel mixture or not enough fuel. This can be caused by vacuum leaks, faulty MAF (Mass Airflow) sensors, or faulty fuel pumps. |
4. Seeking professional help – Once you have or haven’t diagnosed your car trouble code/check engine light, you can always reach out for professional help if things seem or start to seem too technical for your expertise or capabilities. Mechanics can help diagnose and fix your check engine light issue so you can get back to driving as soon as possible. However, buyer beware. It is generally good to be at least knowledgeable on what type of issue you are facing with your car. This helps you have a better understanding of what the situation is so you can potentially avoid any scams. Either getting a free diagnosis from AutoZone or if you already have the means to do it yourself, this can give you a big heads up before you seek professional help.
Storytime: I once had a steady check engine light on my VW CC. I made sure not to panic. I drove carefully to the nearest autozone and got a free scan which told me I had a faulty camshaft position sensor. I went straight to VW got the original part to replace, installed it myself because it was easy, and the check engine light went away. All I spent was my time and the cost of the new part. Otherwise, I could have freaked out, taken the car straight to VW, and paid the price of the diagnostics parts and labor, which was considerably more than I had invested. Knowledge is power.
Safety and Prevention Tips
Driving with the Check Engine Light On – A common way you usually find out that your check engine is on is when you are driving. Continuing to drive while the check engine light is on may not be as bad. However, the check engine light lets you know something is wrong. The quicker you find out what the issue is the better. Continuing to drive while that warning light is on could risk further damage to vital parts of the engine. But don’t panic just yet. As we discussed in the beginning a steady check engine light is like a level 1 warning system to let you know something is wrong.
The engine will still operate but not at 100% of its original manufacturing capacity. Yes, this means you can drive but you should do, is try to find out what’s wrong as soon as possible. A Flashing or red check engine light comes with more seriousness. You should safely try to pull over with your hazard lights on or limit the engine’s throttle to low rpm. In some cases, your vehicle will go into limp mode to try and preserve the engine components. This mode limits your car’s speed, rpm, and gear selection for cautious driving. So yes you can still drive your car with the check engine light on but this driving style should not mirror how you drive as if the check engine light is off. Be cautious, drive safely until you figure out the issue.
Regular Maintenance – Your car works as well as you treat it. One of the keys to a long-lasting engine is how well you keep it lubricated. Inside an engine, there is a lot of potential for metal-on-metal contact. Keeping fresh oil and being on top of your engine’s maintenance schedule can save you a lot of time and money on engine repairs.
Immediate action – When a more serious level check engine light comes on like the flashing or red check engine light, take immediate action. This doesn’t mean panic, but it does mean as soon as safely possible you should pull over and turn the engine off or at the very least limit the engine speed to about 30mph. The flashing or red check engine light indicates a more serious issue that can threaten the engine’s lifespan. That being said your actions should reflect that seriousness.
FAQ’s
- What should I do if my check engine light comes on and off?
As I mentioned above, go through the basics.
- Start by checking for a loose or faulty gas cap.
- If the issue persists, follow the next step below
- Turn the car on and off at least three times to give the electrical system a chance to reset.
- If the issue persists, follow the next step below
- Use an OBD-2 Scanner to figure out what the issue is.
- If the issue persists, follow the next step below
- Once you have figured out the issue and, it is too technical for you seek professional help.
- Can a loose gas cap cause the check engine light to come on?
Loose or faulty gas gaps will cause a steady check engine light to come on. The gas cap is there to perform a seal and keep pressure. If this part is loose or faulty gas vapors get out or unwanted air can get in. The change in pressure will trigger your car’s ECU thus illuminating the check engine light.
- Is it safe to drive with the check engine light on?
It depends on what kind of check engine light. If the check engine light is flashing or red, you would want to pull over as soon as possible. Especially if you’re hearing noises that are outside the norm of your engine running. Some cars will go into limp mode limiting your gear selection, rpm, and speed. This is to preserve the engine from more damage. You can drive with a steady check engine light but you have to be cautious. You’re driving style should not mirror that as if the check engine light is off. Precede cautiously and find out what’s wrong as soon and safely as possible.
- How can I reset the check engine light?
There are a few ways to reset your vehicle’s check engine light. However, if the check engine light comes back you should figure out and fix any underlying issues. Here are a few ways to reset:
Scan tool/OBD-2 – The easiest way is to use a scan tool/OBD-2 scanner. There are a variety of scan tools and free scanning options available. Most come with an option to reset your check engine light once plugged into the car.
Start & Stop Ignition – Turning the ignition on and off at least three times back to back gives your car’s electric system a chance to reset itself, erasing any codes in the process.
Disconnecting the battery – Turn off the engine, remove the negative terminal from the battery, and wait up to 5 minutes before reconnecting. This resets everything. The downside is that it resets everything, so not only the check engine lights but the radio, clocks, etc.
- What are common reasons for the check engine light to come on?
Common reasons for the check engine light to come on are:
- Loose or faulty gas cap
- Faulty Oxygen (O2) sensor
- Mass Airflow sensor (MAF)
- Clogged or faulty catalytic converter
- Worn/Faulty spark plugs or Ignition Coils
