Why Does My Engine Light Keep Coming On And Off? (11 Reasons Why)

It is alarming enough when your check engine light suddenly turns on. Imagine hurrying to the nearest mechanic only to see that the check engine light has suddenly turned off.

Over the next couple of hours, the check engine light may do this as though it is playing with your head, and you want to know what the problem is. Here is what we found!

Why Does My Engine Light Keep Coming On And Off?

1. There Is A Problem With The Check Engine Light

Check engine lights are designed to stay on until the detected problem is fixed. This way, drivers can tell at a glance when their car is experiencing troubles or is functioning well.

However, if the check engine light does not stay on, there might be something wrong with the light itself.

If a mechanic diagnoses your car and everything is in good running order, then consider getting the check engine light fixed to stop the intermittent flashes it makes.

2. Your ECU Is Not Storing Fault Codes

Your electronic control unit (ECU) is supposed to store fault codes that signal the check engine whether to turn on or off.

If the ECU is not storing codes, the check engine receives mixed signals.

Additionally, the ECU could be reset after storing a fault code without using the OBD or diagnostic scanner tool first.

As a result, the check engine is repeatedly alerted there’s a problem and that the problem is fixed, explaining why the check engine light is flashing.

3. There Is An Issue With Your PCM

Your car’s Powertrain Control Module (PCM) is responsible for monitoring and testing the efficiency of your emissions, ignition, and fuel.

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During one of these regular tests the PCM performs, a component can fail a test once or twice, and the PCM will turn the check engine light on to alert you.

However, if the problem is not fixed, but the PCM turns off the check engine light only to turn it on again in the next minute, hour, or drive cycle, your PCM might be faulty.

4. Your PCM Is Doing Its Job Right

It is also possible that your PCM is just doing its job right, which is why it keeps turning your check engine light on and off.

Technically, your PCM keeps testing the systems or components that failed a previous test.

So, if the same system or components pass the test twice in two consecutive drive cycles, the PCM will assume that the problem fixed itself and there is nothing for you to worry about.

5. Your Engine Has Recurring Issues

Your check engine light will turn on for a dozen different reasons, none of which are specified by the system.

That said, if you get one problem fixed and the check engine light turns on again, your PCM is alerting you to another issue you have not addressed.

If your car is suffering from difficult accelerations, loss of fuel economy, and rough idles, it may be best to get your car diagnosed by a reputable mechanic instead of fixing it yourself.

6. You Are Using A Cheap Trouble Code Reader

You Are Using A Cheap Trouble Code Reader

Nowadays, it is easy to be your own mechanic by using diagnostic tools online. After all, it is cheaper than getting your car to the nearest mechanic.

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However, cheap code readers are not as reliable as those used by reputable mechanics. Additionally, it is possible you are not using your code reader correctly.

Oftentimes, high-quality code readers will show pending codes that do not appear in cheap counterparts.

These undetected codes may be the reason your check engine light alerts you to car troubles on and off.

7. There Are Escaping Fuel Vapors

Escaping fuel vapors due to loose or broken gas caps can trigger your check engine light.

Under normal circumstances, the check engine light should remain on until the fuel vapors are no longer leaking.

However, if the fuel vapors are leaking intermittently or the PCM is not detecting it during every test, your check engine light will go on and off.

8. Your Gas Cap Is Not Fitted Correctly

Your check engine light can flash repeatedly for minor reasons like gas caps that are not fitted correctly.

With that, it could be that the cap is worn or not suitable for your car. Either way, the cap can become loose every now and then while you are driving.

Consequently, the check engine light will keep alerting you of the problem whenever the cap becomes loose and turn off again when the cap goes back into place.

9. Your Vacuum Hoses Are Worn

Vacuum hoses are prone to wearing out over time, especially when you fail to maintain them.

In the process of deteriorating, the vacuum hose can show small signs that can trigger your check engine light to flash repeatedly.

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10. Malfunctioning Ignition Coils

When ignition coils encounter problems, it brings to life a host of issues that can keep triggering your check engine light.

Also, you might miss the root of the problem if you are an amateur mechanic or are not using the correct diagnostic tools.

Further, until you fix the ignition coils, the check engine light will keep flashing on and off.

11. Your Spark Plugs Are Faulty

When your ignition coils are acting up, there is a possibility that your spark plugs will follow suit or vice versa.

Additionally, faulty spark plugs and ignition coils have similar symptoms, such as engine misfiring, poor fuel economy, slow acceleration, and engine surging.

Therefore, to stop your check engine light from turning on and off, have your spark plugs either repaired or replaced by a professional.

To know more, you can also read our posts on why your car defrost is not working, why your car is creaking, and why your car is chirping.


Check engine lights can go on and off mainly because of troubles related to your car’s PCM, ECU, and the check engine light itself.

If your system is not detecting or storing faulty codes correctly, your check engine light will repeatedly be tripped off to alert you that something is not going right.

That said, your best choice is to have your car checked by a professional instead of doing it yourself, especially if you do not have the necessary tools and experience.

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