Check engine light is on and off intermittently. Reasons and solutions

The check engine light, a part of the car’s computer

For more than two decades, cars have been equipped with the second generation of OBD diagnostic system known as OBD-II. This is an improved and standardized version of the previous generation. 

The check engine light is part of your vehicle’s On-Board Diagnostic (OBD) system. This system is controlled by an Electronic Control Unit (ECU), which is your vehicle’s computer. 

For example, a sensor or actuator can detect misfire, poor fuel injection, the unusual output voltage to the ignition system, bad fuel pump operation, or any other condition that can affect engine performance or emissions. Your car’s computer (ECU) will first try to correct the problem or wait for several cycles. 

When does the check engine light error keeps going on and off intermittently?

  • Engine problems including worn parts.
  • Fuel system problems.
  • Bad injectors, regulator, or fuel pump.
  • Increased emissions.
  • Problems related to the oxygen sensor, EGR valve, EVAP sensor, and the catalytic converter.
  • Vacuum, hose, or gasket leak problem.
  • Actuators or sensors are giving a bad signal to the Electronic Control Unit.
  • Computer (ECU) malfunctions.
  • Short circuits.
  • Electrical problem, corrosion, or broken wire.
  • Problems related to engine filters caused by dirt or debris, mostly because of poor maintenance.

If the computer is not able to solve a problem or it does not correct it, the ECU will store in its memory a trouble code, that will trigger the check engine light, in most cases, especially when the problem is related to the engine, in your car’s dashboard, and you will be able to read that code when you will connect the OBD to the car’s computer.

When a potential issue that requires additional check is detected, you will see the check engine light come on in different ways.

Intermitent check engine light, it keeps going on and off

This problem can be caused by some failure, either software or on some car parts, but it can also be caused by mistake. In this situations, the check engine error light will flash intermittently, going on and off, stops flashing and then start again. The problem appears only in certain conditions. For example, a loose connector or broken wire keeps connecting and disconnecting as the car goes over bumps on the road, causing the lights to turn on and off for a period of time.

This is one of the easy fixes, but you should consider checking for this issue first and not run to other complicated diagnosis from the beginning.

Check engine light stays always on

If the check engine light stays always, this is typically related to a major issue, but not always. For example, a common cause is a dirty or bad mass airflow (MAF) sensor or bad spark plugs. Because they are not working as they should, it can cause the check engine light to illuminate until you clean or replace the sensor or spark plugs.

This is just an example, because a check engine light can be triggered by numerous car problems.

Check engine light is flashing constantly

If the check engine light is constantly turning on and off, it means you may have a serious problem that can cause serious damage if neglected, and the reasons vary.

For example, one common cause is a misfire, which allows fuel to pass into the exhaust manifold and down into the catalytic converter, where the high temperatures ignite the fuel, leading to serious damage to the exhaust system or catalytic converter. 

Try not to ignore these situations before minor repairs turn into major and expensive ones.

The check engine light is on, but it will turn off itself after a while

Other cases are when the engine light error is on, but it will turn itself off after a certain period of time. The issue here is mostly related to a faulty oxygen sensor that sends to the Electronic Control Unit wrong signals and the ECU understands that there is a problem with that sensor.

The same thing can happen with a bad catalytic converter and other sensors that require cleaning. Clogged or dirty filters also can be responsible for this situation, but also bad electrical wires or worn hoses or gaskets.

Solutions for an intermittent check engine light error

When the check engine light is constantly intermittent or goes on and off after a certain time, you should first check the engine hoses, gaskets, electrical wires, filters, but also the conditions of the spark plugs.

After that, connect the OBD to your car’s computer to see the stored errors. This will give you valuable information about the car’s problem, or at least an idea about the issue.

Car parts like the spark plugs, catalytic converter, MAF sensor, and oxygen sensor have a close connection with the fact that the check engine light keeps going on and off. So it is best to pay close attention to these parts as well.

Modern ECU in cars will store more accurate error readings regarding the check engine light, which makes problems easier to detect and fix. Also, try to avoid checking these errors with aftermarket software and cheap OBD scanners because the readings will not be accurate, and it will cause more harm than good.

How to reset the check engine light indicator

After diagnosing and eliminating the malfunction that caused the light to turn on, it is necessary to delete the fault codes from the computer memory. On older vehicles, you will just need to disconnect the car battery for a few seconds to clear the codes from memory.

However, on newer models, this method is risky, because you will erase the computer system settings.

If you disconnect the battery on some car models, you may notice that the engine is not working quite correctly. In addition, the ECM on some modern vehicles can store DTCs in memory for several days even without battery power. Instead, use the scan tool to reset the error light.


If one of the sensors or other issue is detected that affects the engine operation in any way, this will trigger the check engine light in most cases, and, depending on the problem, this will cause the error light to behave as in the situations presented above.

The car’s computer monitors and is involved in the number of emissions generated during the operation of your vehicle, constantly checking and adjusting the input values ​​for the relevant systems using various sensors.

In modern cars, the ECU has a crucial role in the proper functioning of the engine, because it monitors virtually all major electrical or electronic systems and scans continuously operating parameters reported by numerous sensors and actuators.

However, more involvement of the ECU does not also mean accurate error detection and readings of the check engine light, because it can also have a negative impact leaving room for mistakes.

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