The most common reason behind the check engine light stays on when car is off is a loose or broken gas cap. It can usually be solved easily by tightening the screw on the loose cap or replacing it.
But there are other issues that are also fair common in causing the CEL to come on. They are
- Engine Misfiring.
- Exhaust Leak.
- Failing Catalytic Converter.
- Bad MAF Sensor.
- Malfunctioning Oxygen Sensors.
- Faulty EGR valve.
- Oil Change Issues.
- Ignition Issues.
- Transmission Issues.
- Faulty Thermostat.
- Charging System Issues.
- Damaged ECU.
Keep reading this article to find out all about these CEL issues in detail and learn how you can solve them. After completing this guide, you’d have a clear idea of how to deal with the CEL. Let’s dive in!
Table of Contents
- What Does Blinking And Solid Check Engine Light Mean
- Check Engine Light Stays On When Car Is Off [Reasons + Solutions]
- 1. Loose Gas Cap
- 2. Engine Misfiring
- 3. Exhaust Leak
- 4. Failing Catalytic Converter
- 5. Bad Mass AirFlow Sensor
- 6. Malfunctioning Oxygen Sensor
- 7. Faulty EGR Valve
- 8. Oil Change Issues
- 9. Ignition Issues
- 10. Transmission Issues
- 11. Faulty Thermostat
- 12. Charging System Issues
- 13. Damaged ECU
- What Should You Do When Check Engine Light Comes On?
- How Long Does Check Engine Light Stay On After Repair?
- Will low oil trigger the check engine light?
- Can low-quality gasoline turn on the check engine light?
- Is it safe to drive with a solid check engine light?
- What if the CEL comes on again after going away?
What Does Blinking And Solid Check Engine Light Mean
Before getting to why your car’s CEL stays on, you should understand how serious the threat is. The severity of the check engine light is different based on its color and how it stays on.
It can be triggered by the smallest of issues, like a loose gap. And it can also be a significant problem like ECU failure. So, the range of issues that causes the CEL to come on varies greatly.
If you have a solid check engine light that’s staying on, it means the engine has detected a problem. However, it doesn’t mean it’ll lead to catastrophic engine failure. You can keep driving in this condition if you’re on the road. But keep an eye on the road and drive safely.
Listen for any unusual sounds coming out of the engine. If you hear any noises your car isn’t supposed to make, pull over and ask for help. In some cars, even a solid CEL indicates major issues. If it appears red instead of orange or yellow like it normally does, there could be a big problem with the engine.
So, it’s important to have your car diagnosed even when you have a solid CEL. Otherwise, it can lead to more problems in the long run. All these potential issues if you’re quick to take action.
On the other hand, if you see a flashing or blinking CEL, it’s a strong indicator that there’s a serious issue with your car. Such problems need to be tended to immediately, and you need to get them checked ASAP to prevent major engine damage. It’s not a good idea to drive in this condition.
The bottom line is that no matter whether the CEL is solid or blinking, you should have it checked soon. The only difference is that you can relax a little if it’s constant. Otherwise, you need to have it checked immediately.
Check Engine Light Stays On When Car Is Off [Reasons + Solutions]
Are you wondering why won’t check the engine light to turn off even when the car is off? Normally, it indicates that there’s something with your car. Remember that the ECU stays on after the car is shut down for a while in some vehicles.
It can easily explain why the CEL stays on after you turn off the car. If it goes out after a while, there’s probably nothing to worry about.
But if it stays on for some time, you need to figure out what’s wrong. Here are the common issues behind the CEL coming and staying on:
1. Loose Gas Cap
Most people think that the gas cap doesn’t have a major impact on the engine and it merely keeps the fuel from flashing it. But it does a lot more than that. It also performs other jobs like keeping dust from getting into the fuel tank and harming the engine.
The gas cap also creates a seal, so you hear a hissing sound when you open it. It makes sure that the fuel system in your car operates properly. The seal will be broken if you leave the gas cap open or don’t tighten it correctly.
Not screwing the gas cap tightly is reason enough for the check engine light to pop up on your dash. Besides that, a broken gas cap will also make the CEL come on.
You’re lucky if you’re getting a CEL because of a loose gas cap. It’s one of the easier CEL problems to deal with. You can just tighten the gas cap to solve the issue. The CEL should go away after a while if there are no other issues.
If you have a broken gas cap, replace it with a new one. It shouldn’t cost more than $15 to replace it. That’s how easy it is to get rid of the CEL if the gas cap has been the issue behind it.
2. Engine Misfiring
The spark plugs fire at the wrong time, causing the engine to misfire. If the spark plugs in your vehicle are worn out, it’ll also lead to more hydrocarbon emissions.
As a result, your engine will perform more poorly compared to its normal operating mode. The CEL can get triggered because of it in a few cases.
The simplest solution to this issue is replacing the spark plugs. It’s not so alarming that it needs immediate fixing. But you’ll enjoy much better performance from your engine after replacing the fouled spark plugs.
The good news is that replacing the entire set of spark plugs in your car is cheap. You can get it done for $50 – $100. Make sure you use fuel of good quality to make spark plugs last longer in the future.
3. Exhaust Leak
A leaking exhaust is the number one reason behind the CEL coming on. An exhaust leak will not only damage your car, but it’s also extremely harmful to the environment.
Here are the common symptoms you should be on the lookout for to know if there’s an exhaust leak:
- Louder sound from the exhaust when driving.
- Popping or hissing noises from outside the vehicle.
- Metal vibrations due to escaping air.
- Worse fuel economy as O2 sensors will most likely send incorrect readings.
- Smelling exhaust fumes.
Can an exhaust leak cause check engine light? There should no longer be any doubt about it in your mind. But what causes an exhaust leak? Here are the most likely reasons:
- Corrosion due to being exposed to different weather conditions.
- The exhaust system is being hit when driving through potholes.
- Failing gaskets that allow for an exhaust leak.
- Poor welding job when repairing the exhaust.
Now that you know the reasons behind an exhaust leak causing the check engine light to turn on, you can diagnose the issues behind it. Inspect the exhaust system thoroughly and fix it if you find any problem that causes leaks.
Most likely, you’d find issues with the pipes. You’d have to get certain parts welded by a professional mechanic to seal the leak. The combination of replacing faulty parts and welding will do the job and keep the exhaust gas from leaking in the future.
4. Failing Catalytic Converter
The catalytic converters are essential parts of the exhaust system in all cars. They clean the hazardous gases and convert them into less harmful gases. In simple words, a catalytic converter transforms carbon monoxide into carbon dioxide and reduces emissions.
Unfortunately, they can fail with time and trigger the O2 sensors inside them. When it happens, the check engine light also comes on.
You can easily diagnose if you have failing catalytic converters. Just scan your car’s computer using an OBD-II scanner and check the error codes.
P0420 and P0430 indicate issues with catalytic converters. If you see these error codes, something is wrong with the CC in your car.
There is a chance that you can get rid of the CEL by cleaning the catalytic converter. If it just got clogged and didn’t go bad, cleaning it will do the trick. Even if the CC loses only more than 5% of its efficiency, it’ll trigger the CEL.
So, cleaning it can often solve your problem and get rid of the check engine light. There are many cleaners you can go for, but we recommend going with a pure lacquer thinner as it gives great results.
Put a gallon of lacquer thinner in the gas tank when it’s half full. Drive the car for about 150 miles at high speeds or rev the engine at 2500 RPMs for about half an hour so the cleaner can do its magic.
You can try another thing before repealing it if that doesn’t do the trick. But you’d have to get the CC from your car to clean it using this method. Put some laundry detergent soap, foam it up, and put the CC in it.
Let it sit overnight to get rid of all the impurities. Put it back in and see if the check engine light goes away now. If that also fails, it’s likely that your catalytic converter has gone bad.
You’ll have to replace the catalytic converter in that case. It usually costs between $400 to $1000 to replace it, depending on your vehicle. So, cleaning it is much better than replacing it if you can solve the problem by doing that.
Remember that there’s most likely another issue in the exhaust system that caused the catalytic converter to go bad in the first place. For example, a blown head gasket is a reason behind failing catalytic converters. If there’s coolant leaking into it, that’s a strong sign of a blown head gasket.
So, identify and solve that if you don’t want the new catalytic converter to go bad soon. You can also keep it functioning for longer by taking good care of it and cleaning it at regular intervals.
5. Bad Mass AirFlow Sensor
The MAF sensor calculates how much fuel needs to be mixed with the incoming air so the engine can operate smoothly. It’ll make the CEL turn on it if it detects that the airflow level is more or less than the engine requires.
It can also cause the engine to run rich and burn more fuel than needed. That’ll also lead to more emissions and cause the CEL to turn on.
Firstly, try cleaning the Mass AirFlow Sensor as it’s prone to getting dirty with time. That can get rid of the oil and dirt built up in it. If that doesn’t work, you’d have to replace the faulty MAF sensor to solve this problem.
6. Malfunctioning Oxygen Sensor
An oxygen sensor is a vital component of the engine as it measures the oxygen level before the gas leaves the engine. It helps in maintaining the right air-fuel mixture of the engine.
Even if one of the oxygen sensors in your vehicle goes bad, it can send the wrong signal to the ECU. That’ll mess up the air-fuel ratio, cause the engine to run rich, and trigger the check engine light.
You’ll have to do the same thing with the oxygen sensors as you did with the other sensors. First, you can try cleaning it and see if that solves the issue. You’re going to have to remove the oxygen sensors to clean them.
Once that’s done, here are the steps you can follow to clean it:
- Get a spray bottle that can spray thin and hard. It’s important as you want to clean inside the little holes of the O2 sensors.
- Drench the heads of the O2 sensors for a few hours in the water but make sure that the connector doesn’t get wet.
- Spray every single hole with the spray bottle and get all the dirt out of them.
- Grab a soft paper towel and wipe the sensors until they’re dry.
Now, put the cleaned sensor back in the vehicle and see if that solves the issue. If cleaning the sensors doesn’t work, replace them. Do it quickly, as faulty oxygen sensors can make the catalytic converter become more clogged.
7. Faulty EGR Valve
The Exhaust Gas Recirculation or EGR valve brings down the amount of nitrogen oxide expelled from the engine. The car also remains efficient because of it as it also sends some of the gas back for combustion.
So, it makes sure of two things – the maximum fuel is utilized by your car and the emissions are lowered. Unfortunately, they can get clogged with time and cause the CEL to come on in certain cases.
If the issue isn’t too severe, sometimes you can get away by cleaning the EGR valve. You’d have to remove it, clean it, and put it back in. But more often than not, you’d have to replace it. It’s a good idea to replace it if it has been used for a few years.
8. Oil Change Issues
Are you still wondering why the check engine light stays on after car is off? Making a mistake when changing the oil in your car can also lead to the CEL being turned on.
Even something as simple as a dipstick that’s not seated correctly can cause oil to leak and cause the check engine light to come on. Besides that, there are other issues like low oil pressure and using oil of the wrong grade that can trigger the check engine light.
The correct solution will depend on the issue that causes the CEL to come on. We’ve written a detailed guide on what you can do if your car’s check engine light comes on after an oil change. Check it out to find the appropriate fix for your problems.
9. Ignition Issues
The spark plugs aren’t the only components in the ignition system that can cause the check engine light to come on. For instance, the ignition coil being broken can also trigger the CEL.
Modern engines can have multiple engine coils. If any of them are damaged, you’ll notice symptoms like engine misfiring, shaking, or abruptly turning off.
You’ll have to inspect the ignition system thoroughly and find out if there are any issues with it. Thankfully, you can easily repair ignition coils if they’re the ones at fault here.
10. Transmission Issues
Have you still not figured out why check engine light stays on even after the car is switched off? The next thing you should look into is transmission issues. The transmission system in your car forwards the energy produced by the energy to the wheels.
The two systems are closely related to each other. Issues with the transmission system can lead to an increase in tailpipe emissions. When the ECU of the car detects that, it will turn on the CEL.
A close inspection of the transmission system is required to find out if there are any issues with it. Once you find which component in the powertrain is faulty, you can replace it to get rid of the check engine light.
11. Faulty Thermostat
The thermostat is an important component of your car that regulates the temperature of the engine. Under normal conditions, the spring of the thermostat keeps it closed when the temperature is cold.
It lets the hot water stay in the engine and warm it up. When the engine heats up, the spring opens up and allows water to go into the radiator to cool down the engine.
The operating temperature of your car’s engine should stay pretty much consistent as long as you live in an area that doesn’t have extreme weather conditions. If the engine goes out of normal range, it’s most likely the thermostat that’s causing the issue.
Though thermostats are simple devices, it’s not uncommon for them to go bad from time to time. Both the engine running too hot or too cold can cause the CEL to come on.
The common error code for an engine that’s too cold is P0128. And a P1258 code indicates that it’s too hot. Inspect your car with an OB-II scanner to know if your car has any of these problems.
If the thermostat in your vehicle has gone bad, you’ll have to replace it. It’s not a difficult job to do on your own if you have some experience with automobiles. You can follow these steps to do it:
- Locate the thermostat assembly in your engine.
- Place a pan under it to catch any coolant or antifreeze that may come out of it.
- Take the radiator hose off and remove the bolts of the thermostat assembly.
- Pull out the old thermostat and put the new one in its place.
- Put the thermostat assembly back together and place the radiator hose back on.
That’s how easy it is to replace the thermostat. If you don’t feel comfortable taking on this job yourself, you can always hire a mechanic to do it.
12. Charging System Issues
Your car needs the power to operate. And it comes from the charging system. If you’re still wondering why does my check engine light stay on, the next thing you should check is the charging system in your car.
A weak or corroded battery, loose or corroded terminals, and a malfunctioning alternator can all trigger the CEL. Even a low battery charge can make the check engine light turn on.
Inspect the charging system in your car, starting with the battery. If the terminals of the battery are corroded, clean them. Tighten them if they’re loose. Replace the battery or the alternator if any of them has gone bad.
13. Damaged ECU
It is one of the worst reasons behind the CEL coming on in your car. The ECUs in some cars are designed to stay on for a while after the vehicle is shut off. If that’s the case with you, there’s nothing to worry about. It should go off on its own after a while.
But if the light stays on, it could be an electronic glitch. If you don’t notice any performance issues from your car but still see the CEL, that’s a strong indicator of a malfunctioning ECU.
Remember that seeing the CEL after the car turns off doesn’t necessarily indicate an issue with the ECU. Some cars are designed such that the ECU will run onboard diagnostics when the vehicle has shut off. Consult your owner’s manual to know how it works in your car.
Depending on the model of your car, it may be possible to repair the ECU. You’d have to consult a mechanic for this problem and find out the ideal solution.
The worst-case scenario is that you’ll have to replace it. It’ll easily cost a few thousand bucks and can make your wallet run dry. Unfortunately, you won’t have any other choice if you want to keep driving your car and make the check engine light disappear.
What Should You Do When Check Engine Light Comes On?
Regardless of whether your CEL is solid or blinking, it indicates that there is an issue with your car. The ideal thing you can do is to take it to a repair shop as early as possible and have it diagnosed.
Having the ECU scanned with an OBD-II scanner will reveal the issue behind it. The systems in your car are more electronically measured than you may think. So, when you see an error code pop up, you know where to look.
Keep in mind that the results of the scan aren’t a perfect diagnosis. It’s only a guideline of where you should start looking. Sometimes, the issues will be incredibly easy to solve. During other times, it’ll take a lot of work to get rid of the check engine light.
Either way, you should get it checked out as fast as you can to keep your conscience clear. When you fix the problem and drive your car for a while after that, the CEL will go away.
How Long Does Check Engine Light Stay On After Repair?
So, you’ve addressed the issue that caused the CEL to come on. Will check engine light turn off after repair is done? Unfortunately, the answer is no and it doesn’t quite work like that.
Your car needs to complete a certain number of cycles for the CEL to go off even after the repair. The ECU of the car needs this time to get adjusted to this condition.
Normally, the engine light should be reset after you complete 10 to 20 cycles. The light should go away on its own. So, you’d most probably have to drive for close to 50 to 100 miles for the CEL to go away.
If you don’t have the time to wait for that, you can expedite this process if you have an OBD-II scanner. You can also take it to a mechanic. Just have your car inspected for codes. If an error code appears, unfortunately, that means there’s still an issue with the engine.
You’d have to get it fixed, and then think about getting rid of the CEL warning. But if no error codes pop up, you could be able to clear the check engine light depending on the status of the car.
If the scanner throws the “Not Applicable” status, it means that the status of the engine cannot be checked. “Not Ready” means the ECU of the car needs time to reset, and you won’t be able to clear the CEL here. But if it says “Ready,” the CEL will turn off, and the car will operate without any issues.
Will low oil trigger the check engine light?
Driving with low oil is a serious risk to yourself and your vehicle. You should never it. However, it won’t trigger the CEL light. But it will trigger the oil light on your dash.
Can low-quality gasoline turn on the check engine light?
It’s not common, but using low-quality gasoline can trigger the CEL if you do it for a long period. Using bad fuel will clog the fuel injectors and turn on the dreaded check engine light.
Is it safe to drive with a solid check engine light?
If you keep your vehicle under close inspection, it’s normally safe to drive with a solid or constant CEL. But keep in mind that you shouldn’t do it if the solid CEL is red or orange.
What if the CEL comes on again after going away?
If the check engine light comes on after disappearing for a while, it means that your vehicle needs additional repairs. Multiple issues together can also trigger the CEL. In that case, all the issues need to be solved.
Why does the check engine light stay on when the car is off? By now, you should know all about it after completing this guide. Hopefully, it’ll be one of the simpler issues in your case like a loose gas cap or fouled spark plugs.
If it’s due to another issue, you’d have to spend a fair bit of time and energy to diagnose and rectify the problem. Let us know what turned out to be the main issue in your case. Comment below if you have any questions about why check engine lights come on.
About John M
John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.