Why Does My Check Engine Light Comes ON And OFF [Causes and Fixes]

You are driving down the road, and the engine light comes on suddenly. It then disappears just as it appeared. It leaves you worried as it keeps flashing on and off. What causes it?

Check engine lights come on for many reasons. But if it flashes on and off, it can be due to the following:

  • Loose gas cap
  • Faulty oxygen sensor
  • Bad spark plugs
  • Faulty mass airflow sensor
  • Faulty EGR valves

Regardless, a check engine light shows a fault or problem with any component related to the powertrain. It can be caused by something as simple as a loose gas cap or as complex as a crankshaft and camshaft misalignment.

Read on for detailed information about the cause of the check engine light coming on and off and how to diagnose and fix it.

What is Check Engine Light?

A check engine light is usually a yellow symbol of the engine block that appears on the dashboard if there is a problem with the powertrain. That includes the engine, transmission, and related components.

The check engine light can be orange or red if the fault is severe and can cause engine damage.

Check engine lights are one of the most misunderstood dashboard symbols. That is because many things can cause it, which vary from one car brand and model to the next.

It comes on to show a fault that interferes with the engine performance and emissions. As mentioned, it can be something as simple as a loose gas cap. In that case, you can continue driving your car for a while until you get the gas cap checked.

However, the check engine light can also come on due to an active fault code that indicates a serious fault or failure. That can be a faulty timing belt, spark plugs, crankshaft position sensor, and many other engine components.

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Since you cannot know what causes the check engine light to come on without diagnosis, check it when you have the opportunity.  

Solid Vs. Flashing Check Engine Light Meaning

Check engine light is often a solid yellow color. But what if it flashes on and off? Some car brands and models flash the check engine light to warn the driver of a serious fault that requires immediate attention.

The check engine light can remain solid but change color to orange or red if the onboard computer determines the fault is serious. Whichever the case, act immediately if the check engine light flashes or changes color. Driving any further can cause severe engine damage.

Why Does Check Engine Light Come ON and OFF Intermittently?

As mentioned, the causes of check engine lights coming on vary for different car models and brands. But if it comes on and off intermittently, it can be due to one of the following reasons:

1. Loose Gas Cap

Loose Gas Cap

A loose gas cap is one of the most common reasons to make the check engine light keeps flashing on and off. That is because it lets fuel vapor escape into the atmosphere.

A gas cap is a vital component of the fuel delivery system. It regulates the fuel pressure in the tank and effectively prevents the fumes from escaping. If it is loose, it fails to do its designated work.

If your check engine light appears immediately after refilling at a gas station. The gas cap is the likely cause. Pull over when it is safe to check it.

And if something prevents you from tightening the cap as it should, get it fixed or buy a replacement.

2. Faulty Oxygen Sensor

Faulty Oxygen Sensor

If it is not a loose gas cap causing your check engine light to flash, it can be due to a faulty oxygen sensor. The sensor ensures the engine gets enough oxygen to efficiently burn fuel.

A faulty oxygen sensor gives a mixed signal, making your car not know the amount of fuel to deliver. That can cause poor fuel economy and increased emissions.

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The oxygen sensor can fail intermittently, which will cause your check engine light to sometimes turn on or off.

Most OBD2 scanners will read the active code for the oxygen sensor to pinpoint the problem. You should diagnose your car to confirm the cause.

3. Bad Spark Plugs

Bad Spark Plugs

Spark plugs are responsible for igniting the air-fuel mixture in the combustion cylinders. If you drive a gasoline-powered car, it can also be a possible cause if it is faulty.

If you have noticed the car is not starting as fast or smoothly as it used to, spark plugs are likely the culprit. Faulty spark plugs will cause misfiring, increased fuel economy, and loss of acceleration. Overall, there will be a decreased engine performance.

Bad spark plugs are considered serious engine faults, which will trigger the check engine light. You can still drive your car with them, but you will realize the reduction in overall performance.

4. Faulty Mass Airflow Sensor

Faulty Mass Airflow Sensor

Air is forced into the engine through the air filter due to the vacuum created in the engine block. This air entering the engine block should be accurate, not too much or insufficient. The mass airflow sensor ensures the engine gets sufficient air.

If the mass airflow sensor is faulty, it becomes impossible to control the amount of air entering the engine. That may cause the check engine light to come on and off or flash.

5. Faulty EGR Valves

Faulty EGR Valves

The exhaust gas recirculation (EGR) valves are prone to failure. It can be a culprit if your check engine light comes on and off intermittently.

EGR valves help to regulate the amount of nitrogen oxide that goes into the environment from your car’s engine. Unfortunately, the valves can get clogged over time and eventually fail.  

Most Common Check Engine Light Codes

As mentioned, you check engine lights can come on for various reasons. Many possibilities exist, but we have the most common causes. These will trigger diagnostic trouble codes mechanics use to determine the cause.

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The following are the most common DTCs that turn the check engine light on:

1. P0171 and P0175

These DTCs are related to the oxygen sensor fault. They indicate a lean fuel mixture, which can only result if your oxygen sensors are not working correctly.

2. P0401 Code

This DTC is set if there is a problem with the EGR system. It can point to a possible EGR valve failure since the system cannot accurately recirculate the exhaust gas to reduce emissions.

3. P0300 and P0305 DTCs

These two DTCs are set when your engine is misfiring. Engine misfires can be caused by many failing components, including spark plugs and ignition coils.

4. Po0420 and P0430 DTCs

These codes become active when there is a problem with the catalytic converter system. As the name suggests, the catalytic converter changes the harmful pollutants from the engine to their safe forms before being released into the atmosphere.

5. P0411, P0440, P0442, P0446, and P0455 DTCs

These DTCs are for the evaporative system. This system prevents gasoline fumes from escaping into the atmosphere. They point at failing components, such as a fuel tank, gas cap, fuel tank pressure sensor, Evap canister, and series of tubes and valves.

How to Diagnose and Fix Check Engine Light

Diagnosing the check engine light involves checking the stored diagnostic trouble code with an OBD2 scanner. It will help narrow down the process and know the system component to check.

Once you obtain the code, you can perform the following check:

  • Check evaporative system components, including the gas cap. There may be a leak that allows the gasoline vapor to escape into the atmosphere.
  • Inspect the spark plugs and ignition coils. These can cause misfiring. Replace them if damaged or clean the spark plugs if there are carbon deposits.
  • Check the air filter, oxygen sensor, and mass airflow sensor. Replace dirty air filters or faulty sensors to allow the correct air quantity into the engine.
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Your check engine light can also be on and remain solid. This has myriad causes, which you can determine from the stored code. Follow the diagnosis procedure for each DTC to solve the problem.

Once you solve the problem, you can erase the error code from the computer memory using an OBD2 scanner. Alternatively, you can let the car rescan itself the next time you start the engine and clear the code once it confirms the problem is solved.  

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What does a solid yellow check engine light mean?

A solid yellow check engine light is the most common and means many things. It is turned on if there is a problem with the powertrain. So, the best step is scanning the car’s computer to get the stored code, which tells you why it comes on.

Why is my engine light on, but nothing seems wrong?

Your car may still drive well with an unnoticeable drop in engine performance, but that does not mean nothing is wrong with your vehicle. There is a minor cause, which you should check and fix before it causes severe damage.

Does the check engine light come on for an oil change?

No. Low engine oil is a severe engine condition that needs immediate attention yet does not usually trigger a check engine light. That is because the low engine oil dashboard symbol is dedicated to it. If you see a red light resembling an old-school can with liquid dripping, your engine oil should be topped up.

What to do when the check engine light comes on?

It depends on the severity of the problem. If your car still drives well and the check engine light is solid yellow, you don’t have to pull over immediately. The car is still safe to drive, but let it be checked as soon as possible.

But if the check engine is orange, red, or flashing on and off, pull over when it is safe and let the car be checked by an experienced mechanic. It shows a severe engine problem that requires immediate attention.

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How long can I drive with the check engine light?

It depends on what caused the check engine light to come on. If the problem is less severe, you can drive for more hours without pulling over. But if it is more serious, as you will notice by the changed car response, don’t drive any further. Let the car be fixed first!

Can you sell a car with a check engine light on?

Yes. You can sell your car with a check engine light, but it won’t be easy finding a buyer for a vehicle with an engine problem. It will also lower the value of your car. So, fix it before listing the car for sale.

Final Remarks

Check engine light is one of the most misunderstood dashboard symbols because of the many causes. But if it flashes on and off, the problem can be narrowed to bad spark plugs, loose gas caps, faulty mass airflow and oxygen sensors, and EGR valves.

Regardless of the cause, take your vehicle to the dealership or call a mechanic to check your car if you see the check engine light. You can also do it yourself if you are a DIYer since some causes are easy to fix.

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