Sometimes the check engine light (CEL) can pop on due to a minor fix that you can easily fix without going to a mechanic. A loose gas cap is one such minor issue that could trigger a check engine light. But how true is this?
Can a loose gas cap cause a check engine light? YES, a loose gas cap is one of the most common for the check engine light to come on. When not properly tightened in place, the gas cap causes harmful fumes to escape, thus triggering the check engine light.
Below, we share full details on why a loose gas cap causes the engine check light to come on, what error code it throws, how to reset the engine light for a loose gas cap, and so much more.
Can a loose gas cap cause a check engine light?
As we have just stated above, a loose gap is one of the most unexpected causes for the check engine light to come on. Most cars now run periodic tightness checks on the fuel system as a way to ensure no fuel leaks are coming from your car.
And if your gas cap isn’t properly put on, your car will most definitely fail the pressure check—causing the CEL light to illuminate on your dash.
Luckily, this minor issue is an easy fix and simply requires tightening the gas cap. You just need to remove the gas cap and put it back on correctly. Turn it until you hear 3-4 clicks.
The issue of a loose gas cap usually happens at the gas station when you forget to properly tighten the gas cap after getting gas. So, if your check engine light comes on after getting gas, a loose gas should be the first place you’d want to look into.
So, what does a loose gas cap have to do with the CEL coming on? You may wonder…
You see, the gas is essentially part of your car’s EVAP (Evaporative Emission Control) system. This system is part of the emission system that ensures fuel vapors don’t get leaked into the air and cause pollution.
Now, the gas cap plays a significant role in keeping the system sealed, thus keeping gas vapors from escaping your car fuel tank. If the cap is loose, it means the seal is broken, and the check engine light will be triggered.
A loose gas cap also prevents the fuel system from pressurizing properly. The vehicle system will detect a pressure loss when the gas cap is loose or missing and consequently trigger the check engine light.
If the check engine light is on and your car is running smoothly, try removing the gas cap and putting it back on correctly. Turn it until you hear 3-4 clicks.
How to tell if a loose gas cap is behind the check engine light coming on?
Loose gas cap is just one reason for CEL to come on, and there are many other causes for your vehicle computer to activate the engine warning light.
So, how do you confirm that a gas cap that’s not properly tightened is indeed the one behind CEL coming on?
The BEST way to determine if you have a loose gas capo causing check engine light is to use an OBD II code reader to retrieve the diagnostic trouble codes from your vehicle system memory. You can also have a professional retrieve the codes for you.
If you indeed have a loose gas cap, the PCM memory could throw one of these trouble codes:
- Trouble code P0455: this means “Evaporative Emission System Leak Detected (Large Leak)”
- Trouble code P0457: this means “Evaporative Emission System Leak Detected (Fuel Cap Loose/Off)”.
Sometimes you may find that the gas cap isn’t loose but it is bad. This is another reason to warrant the trouble codes and make the check engine light come on.
If you find that your gas cap fails to tighten correctly, inspect its gasket to see if it’s missing or damaged. If the O-ring is old, dried, or cracked, it may not create a tight seal and will make the cap stay loose. If you own an old car/truck, ensure its metal gas cap isn’t bent or damaged.
The best remedy for a bad gas cap is replacing it with a new one. They’re relatively inexpensive (they cost around $15 to $30 for most car models) and widely available, so finding one for your car will not be a hassle.
How do you reset check engine light after loose gas cap?
After confirming that you’re dealing with a loose or faulty gas cap, we recommend you first check it and fix it before anything else, else the check engine will come back on.
Simply remove the gas cap and tighten it correctly into place (until it clicks). Or, if it is faulty, replace it with a new one.
Afterward, you can use a scanning tool to help you clear the error codes from your car’s computer memory. This is because the codes won’t go away on their own and the light may take longer to clear off, so you’ll need to clear them and reset the CEL.
If you do have a scanning tool, you can take your car to a mechanic or a local auto parts shop to have them clear the codes for you.
You can also use the disconnect battery method to help you reset check engine light after loose gas cap.
You can then continue driving your car as you normally do and see if the light will come back on.
If it doesn’t come on again after up to a week, then it means fixing the gas cap did solve the problem and there’s nothing else to worry about.
How long does it take for check engine light to go off after tightening gas cap?
The check engine light usually goes off after you drive your car for a few minutes if a loose gas cap was the reason behind the CEL.
However, some car owners, have explained their lights hang around for days before going off.
In some cases, the light may turn off after around 50 to 100 miles after tightening the gas cap, depending on your car model.
If the light comes on and then goes off again after tightening the gas cap, then it means your gas cap is too loose and won’t hang on tight. Consider replacing it with a new one to see if the light goes off for good.
Why is the check engine light still on after tightening gas cap?
If the check engine light is still on after tightening the gas cap, then it means the gas cap is too loose and not forming a tight seal in the fuel system.
In other words, you could be having a bad gas cap that doesn’t form a tight seal, causing the check engine light to come on. Inspect your gas cap gasket for damages that could make it loose.
Remember, without an air-tight seal, your entire fuel system is exposed to grime and dirt (this affects how your car runs) while allowing fuel vapors to escape. This would trigger the emissions systems to throw error codes and activate the check engine light.
If you have a faulty gas cap, replacing it with a new model will fix the problem and clear off the CEL most of the time. Gas caps are affordable, so you don’t even need to visit a mechanic.
Why is the check engine light still on after replacing gas cap?
In case you replaced a gas cap and the check engine light is still on, then the fuel filler neck itself (the neck is where you attach the gas can when refueling your car) could be damaged.
If the neck has suffered abnormal damage and wear, not even a new gas cap can properly seal it.
If you can’t find any problems with the gas cap, you should inspect the filler housing. If it is dented or stripped, then the new gas cap won’t fit into the fuel tank and your car won’t be able to maintain internal pressure.
Damaged filler housing is a more serious problem and you should take your car to a mechanic to have it fixed.
Replacing the old or broken filler neck will help the gas cap form a tight seal and prevent fuel vapor escape, making the light go away.
Can a loose gas cap cause a cylinder misfire?
A loose gas can cause cylinder misfire. And misfires are a common cause for check engine light to come on in a flashing mode.
When the gasp gets loose, it lets in contaminants into your car’s fuel tank, through a process called cold running. When this occurs, the dust and moisture from the gas evaporation may result in engine misfires.
Your car may also experience rough idling or stalling when you start the engine and there’s too much moisture in its ignition system.
Loose gas cap also affects the fuel injection pressure, which may also trigger a misfire. This is because a misfire can occur due to the vehicle engine not creating the necessary combustion required to drive the cylinder.
When the fuel injection system is unable to properly create a balanced mixture of gasoline and air, the engine will not be able to produce the needed combustion for driving the cylinders, a misfire occurs and the CEL will be activated.
Although a loose gas cap can cause engine misfire, it’s important to keep in mind that many other issues can cause misfires as well, and you’ll most likely catch the loose gas cap before it creates a misfire.
So, you may want to look into other common causes of engine misfires like bad spark plugs, sensor problems, a transmission leak, and even a faulty fuel pump.If your check engine light is on because of a loose gas cap, you might find our articles on related topics helpful. If your check engine light went off after a week or so, it might be a temporary issue. Our article on why did the check engine light go off after a week? explains some of the reasons why the light might turn off on its own and what you can do to prevent it from happening again. On the other hand, if you’re worried about the cost of getting a diagnostic test done for your check engine light, our article on how much does a check engine light diagnostic cost? provides a comprehensive guide on the average cost of diagnostic tests and how to save money. With our expert advice, you’ll be able to diagnose and fix your check engine light issues without breaking the bank.
Frequently Asked Questions
Yes, the check engine light usually clears itself after a few minutes after you tighten the gas cap, though it may take longer depending on the vehicle model. If the light doesn’t seem to go away after tightening the gas cap, the gas cap could be faulty and you’ll need to replace it with a new one to make the light go away.
Driving with a loose gas cap isn’t recommended because it allows easy entry of dust and dirt into the fuel tank, which may then cause damage to the engine.
It will also damage the car fuel injection and emissions system with time, and if your car computer picks up incorrect emission readings, it will try adjusting using wrong data which can end up damaging the sensors, hoses, and other essential parts.
Gasoline vapors will escape into the atmosphere when the gas cap is loose, leading to environmental pollution.
Not to forget, you’ll be inhaling the toxic fumes emitted by gasoline. Long-term exposure to these fumes can lead to heart disease and brain damage.
A loose or faulty gas cap will throw a P0455 or a P0457 trouble code to your car’s ECM. These codes usually mean that your vehicle is suffering from a massive leak or loose gas cap issue. Fixing the gas cap and clearing the codes with an OBD II scanner will help clear the check engine light.
That’s it for our guide on whether a loose gas cap can cause the engine warning light. A loose gas is a potential trigger for the check engine light. This is because the cap is unable to create a tight seal, causing fuel vapors to escape from your car fuel tank. These vapors will then produce a fault code in the emissions system which could trigger the check engine light. A faulty or damaged gas cap will also cause the check engine light to come on.
Luckily, CEL caused by a loose gas cap is an easy fix and you don’t need to pay your mechanic a visit. Simply tighten the gas cap correctly until it clicks 3-4 times. If the cap is cracked or damaged, replace it with a new one. Afterward, turn on your car and the engine light will go off after a few minutes of driving or after driving for 50 to 100 miles, depending on your car model.