Check Engine Light Turned Off by Itself: Why It Happens

A check engine light turned off by itself could signify a couple of reasons which may require immediate attention. This is because an illuminated check engine signal indicates that there is an ongoing issue with the car but can’t identify the exact issue, so you will need to bring the vehicle to a professional mechanic to fix the problem.

Check Engine Light Turned Off by Itself

With the help of an OBD scanner, you can identify the problem and fix it and this could cause the light to go off. However, this is only one reason why the light would turn off by itself, so read this complete guide until the end to identify the rest of the reasons.

Why Did Your Check Engine Light Turn Off by Itself?

Your check engine light turned off by itself because the problem with the engine has been fixed. The light may also go off because your car performed a few tests on the engine, which it passed. However, you still have to visit the mechanic to make sure all is fine.

Light Turned Off After a Week

What happens when the check engine light went off after a week? Sometimes the check engine or service engine soon signal illuminates and then goes off on its own after a week when you’ve not even visited the mechanic yet.

Light Turned Off After a Week

Well, the engine control unit (ECU) performs diagnostics tests whenever you turn on the engine to ensure all the components are in prime condition and working properly. Thus, during the first drive when the light turned on, the engine control unit performed a few tests on the engine which failed. However, during the last drive before you decided to go to the mechanic, the diagnostic tests were performed again and this time the car passed.

The cause could be a loose fuel cap which you may have tightened after you bought gas a week later. A loose gas cap can cause the gas to escape and let in dust and dirt, which can contaminate the fuel and hurt the engine. Thus, when a gas cap is not properly tightened, the check engine sign will illuminate, and when it is firmly closed the light will go off.

However, we recommend that you still take the vehicle to the mechanic for full diagnostics to ensure the engine is working well. The light came on because the ECU detected a problem, therefore, it is best if you have an expert take a look before concluding that all is well. 

Light Turned Off After Diagnostics

How about when the check engine light went off after diagnostics? When the light goes off after diagnostics, it is an indication that all is well and there’s no cause for alarm. However, this may not happen immediately as the engine has to go through a few cycles before the light goes off. A cycle is a time between turning the engine on, running/driving around for some time, and turning it off. According to the experts, the light goes off after an average of 20 cycles.

Therefore, if the light still remains after diagnostics and fixing the problem, don’t fret. You can drive the car after your mechanic has given you the green light, while you wait for the engine light to go off. You can also reset the light manually with an OBDII scanner if you don’t have the patience to wait until the 20th cycle. Whatever method you choose, remember to seek the consent of an expert before your car hits the road to avoid any negative occurrences.

Light Turned Off After a Month

What happens when the check engine light went off after a month? Well, if you face a situation where the light comes on for a month and then goes off by itself, then the sensor might be damaged. You’ll need to have it replaced to see if the check engine sign will illuminate again. In most cases, the problem might not be the sensor and replacing it would be a waste of money. The ideal thing to do is to scan for error codes as soon as the check light engine comes on and not wait for a whole month.

Light Turned Off After a Month

Promptly checking and fixing what caused the light to illuminate can save your engine and even your life. Once the issue is solved, the light will go off and you won’t have to worry about anything damaging the engine.

Light Turned Off by Itself Because of O2 Sensor

You might be experiencing the check engine light went off by itself O2 sensor. The O2 sensor, which is also known as the oxygen sensor, monitors the amount of unburnt oxygen that leaves the exhaust. This allows the ECU to know whether the oxygen is burning properly in the combustion chamber. It also monitors the catalytic converter to ensure it is working properly. With this information, the ECU makes appropriate adjustments so that the vehicle can make optimum use of the oxygen that enters the engine. 

Therefore, if the O2 sensor is damaged, the light will come on. If the light goes off when you’ve not repaired the O2 sensors, chances are that the sensors have developed a fault. They may need to be replaced to ensure the cat converter is not damaged. 

Also, a damaged mass airflow sensor (MAF) can cause the light to illuminate. This sensor measures the amount of air that goes into the engine for combustion, thus if it is faulty, there may be too much or too little air to combust. A faulty MAF needs replacement to ensure it works well.

Light Turned Off Automatically After Repair

What happens when the check engine light went off automatically after repair? It should then not come as a surprise that the check engine light goes off after the problem has been fixed. After all, the light is an indicator that the car has developed a fault, therefore, once the fault is out of the way, it is only logical that the light goes off.

If the light stays on even after repairs have been carried out, it means that the system has detected another fault. Let your mechanic thoroughly check your engine to eliminate all faults.

Light Turned Off by Itself Due To Catalytic Converter Failure

How about when the check engine light went off by itself catalytic converter failure? As mentioned earlier, the ECU monitors the catalytic converter to make sure it is burning harmful gases. If the converter becomes faulty, the sensors will signal the ECU, which, in turn, will trigger the check engine light.

However, if the light goes off by itself after you’ve repaired the converter then that is a good sign. Also, when the converter is under a lot of stress due to many occasions of stop-and-go, then the check engine sign may illuminate. This problem can be remedied if the pressure on the converter is reduced.

Light Turned On Then off While Driving

Why did your check engine light came on then went off while driving? When the light goes on and comes on while you’re driving, then the engine thermostat could be faulty. The engine thermostat is the device that allows the coolant to circulate and keep the engine running at optimum temperature. If the engine thermostat is faulty, it can’t properly regulate the engine’s temperature, causing it to overheat and return to normal temperature concurrently. This could be responsible for the on-and-off nature of the check engine light during driving.

Light Turned On Then off While Driving

You can have an expert look at the thermostat and replace it to keep the light off. Also, let the expert check the engine for other signals that can illuminate the light and have them fixed. If you don’t find any fault and the light is still on, then try an engine light reset to fix the problem.

Another problem could be the EGR (exhaust gas recirculation) valve, which reduces emissions by sending back exhaust into the combustion chamber for burning. However, when the EGR accumulates soot and cannot function properly, it may send intermittent signals which may cause the light to fluctuate.

– Malfunction Indicator Light Turning Off by Itself

Is it normal for a malfunction indicator light to turn off by itself? Yes, but only if the problem that caused it to illuminate in the first place is solved. If not, the light will persist until the problem is fixed. If, after checking, you don’t find any fault, you can try resetting the light.

For example, the check engine light will automatically turn off after repair, provided there’s no underlying problem that needs to be fixed. Ensure the mechanic does a thorough check to find and eliminate all faults. 

What Are the Common Codes that Can Illuminate the Engine Light?

The common codes that can illuminate the engine light include P0171 to P0175, P0401, P0300 to P0305, P0420, and P0430. When the OBDII scanner detects a fault, it translates it into codes that indicate exactly where the fault can be located in the vehicle.

The check engine light is a colored indicator that is found on the dashboard of vehicles which illuminates whenever the car’s engine develops a fault. It is also known as the malfunction indicator lamp (MIL) and comes in yellow, orange and amber, depending on the producer.

The malfunction indicator light works with the Engine Control Unit, which has several sensors throughout the vehicle. When the sensors pick up a fault, they communicate to the ECU which, in turn, illuminates the malfunction light indicator. The fault can then be diagnosed with an OBDII scanner which is plugged into the car’s diagnostic port. 

Here are a few common codes that the OBDII scanner will display to warn you about an engine problem:

– P0171 – P0175

These codes indicate that there is an oxygen sensor failure, however, to know the exact fault, look at the last number. Codes P0171, P0172, P0173 and P0174 indicate that the air-fuel mixture isn’t enough and Code P0175 shows that the air-fuel mixture is too much.

– P0401

When you see this code, it means something is wrong with the Exhaustive Gas Recirculation valve.

Common Code P0401

It simply means that the EGR is not circulating enough exhaust gas, thus increasing the car’s emissions. 

– P0300 – P0305

These are engine misfiring codes that should be attended to immediately if the engine is to be saved. It means one of the cylinders in the engine is not firing properly, probably due to damaged spark plugs or any component of the fuel ignition or fuel injection systems. The last number in the code tells the specific cylinder misfiring. For instance, a P0304 only means that the 4th cylinder is the one misfiring.

– P0420, P0430

Any of these codes is indicative of a problem with the cat converter, which transforms harmful gases into less harmful ones. It shows that the ECU has determined that the converters are not in optimum condition and this will illuminate the malfunction indicator light.

Can the Check Engine Light Turning Off by Itself Be Related to the Car Shutting off When the AC Is Turned On?

It can be concerning when the check engine light in your car turns off on its own, especially if it coincides with the vehicle shutting off when the air conditioning is turned on. This phenomenon, often referred to as “car shuts off with AC on,” might indicate a potential underlying issue with the electrical system, cooling system, or engine sensors. It is advisable to consult a professional mechanic to diagnose and address any problems to ensure safe and reliable driving.


We’ve covered the various reasons why the malfunction indicator light will turn on and off on its own and the common codes associated with it. Here is a recap of all that we’ve read:

  • The malfunction indicator light is indicative of a fault in the car and therefore, if it goes off by itself, then that means the problem has been fixed.
  • Sometimes the problem could be a faulty thermostat that is causing the light to illuminate.
  • Whenever the malfunction indicator light comes on, you can use the OBII scanner to diagnose where the fault is and allow a mechanic to fix it.
  • The malfunction light indicator is connected to the Engine Control Unit, which monitors the entire vehicle to ensure all parts are working perfectly.
  • It works by illuminating whenever the ECU detects a problem with the engine and will only turn off once the issue is fixed.

However, if all has been fixed but the light refuses to go off, then it is time for a reset. Resetting will allow the light to function properly, but don’t try resetting until you’re sure that all the problems have been solved.

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