Gas Cap Throwing a Check Engine Light? [Diagnose & Fix]


Deciphering the code

Some drivers might confuse the service required or maintenance required light on the gauge cluster for the check engine light. These warning lights are unrelated. The service required light just means the car is due for an oil change or other routine care. It is not an indicator of trouble like the check engine light is.

Your local mechanic can usually diagnose the problem for about $75. But there’s a way to preview what the problem might be. Do-it-yourselfers can buy inexpensive code readers from an auto parts store or online that connect to the onboard diagnostics (OBD) port and search for the code’s meaning on websites such as Engine Light Help. Modern systems will display the code in an app on your smartphone.


No. 4: Oxygen Sensor

The oxygen sensor plays an important role in your vehicle’s operation because it monitors the unburned oxygen that comes out of your exhaust. That allows it to help your car accurately monitor fuel consumption. A faulty sensor can greatly reduce your fuel efficiency and can even cause damage to your spark plugs and catalytic converter.

Don’t ignore that light

Mazor says that some people freak out when they see the check engine light. “They just put a piece of black tape over the dashboard light and keep driving,” he said. But Mazor adds it’s important to address problems indicated by the light promptly. Ignoring them could lead to larger, costlier problems later.

If the light comes on, Mazor suggests the driver check the gas cap. A loose gas cap sends an error message to the car’s computer, reporting a leak in the vapor recovery system, which is one aspect of a car’s emissions system. If the fuel cap is loose, tighten it and continue driving. Even so, it will take some time for the light to go off, he says.

What should you do if the check engine light comes on and it’s steady rather than flashing? The most obvious answer is to get the engine checked by a mechanic. But many people do nothing, perhaps fearing an expensive repair bill. Some drivers with older vehicles may want to squeeze out as many remaining miles as possible without visiting a service garage. But before they can pass their state’s vehicle inspection, they have to get the light turned off. And a state inspection is a good motivator for dealing with the problem. Ultimately, the engine and the emission control system are so interlinked that the health of the emission control system is a good indication of the general health of the car’s engine.

Vacuum leak

Every car has a vacuum system that performs a wide variety of functions. The brake booster is vacuum-operated, and the vacuum system also helps lower emissions by routing the fumes as gasoline evaporates through the engine. If your car’s idle begins to surge or settles at an unusually high rpm, a vacuum leak could be the culprit.

Vacuum hoses can dry out and crack as they age, especially if they’re exposed to intense heat or extreme cold. This is the most common cause of vacuum leaks. Other common issues include cracked fittings and loose connections. Vacuum lines cost just a few bucks each, but tracing the source of the leak can be time-consuming — and expensive if you’re not performing the work yourself.

Dead battery

The battery is as simple as it is important; without it, your car won’t start. Today’s batteries last much longer than before and they’re maintenance-free. The price of a new one depends on the type of car you drive, but plan on spending at least $100 for a quality battery.

Changing or charging a battery on your own is a relatively easy task, but keep in mind that in some late-model cars, it’s buried under countless plastic covers, and it might be a little difficult to access. Also, note that disconnecting the battery will often reset your stereo system. If you don’t have the code, ask your local dealer for it before you unbolt the positive and negative terminals. Otherwise, you’ll be driving in silence.

Failing Engine Control Module

The engine computer is commonly referred to as the:

  • Engine Control Module (ECM), or
  • Engine Control Unit (ECU), or
  • Powertrain Control Module (PCM)

The ECM controls all the essential functions of your vehicle including engine performance and the drivability functions.

When the ECM is failing, it will trigger the check engine light to alert you of a potential issue with its sensor or circuits.

Other possible signs of a faulty ECM are:

  • Engine stalling or misfiring
  • Engine performance issue such as throwing off timing and the engine fuel settings
  • Car not starting or difficult to start

How to fix a failing Engine Control Module?

The ECM plays a major role in the performance of your car engine. Any issue with it can largely affect the overall performance of your car.

Therefore, have your vehicle inspected by a certified technician because the ECM is quite sophisticated and complicated.

Sensors Can Fail and Report False Codes

Your car has sensors for many components. Most are related to emissions control (see below for more about the tire pressure warning light on newer cars).

Sometimes a sensor will fail or get stuck and report a false code. An honest mechanic will tell you that. Replacing a sensor is not that expensive.

You can do your own cheap engine diagnostics by using one of these low-cost code scanners. That will help you discover why your check engine light is on. But knowing if it's a sensor problem takes some extra work, as I'll explain.

How to Read Your Code Scanner

This scanner that I use doesn't require me to look up the meaning of the codes. It shows the description of the codes on the screen. It also lets you reset the check engine light by clearing the codes from the computer.Most diagnostic scanners offer similar features. Actron has versions that also read OBD-I codes in older vehicles with an optional cable. By reading the codes myself, I was able to see that my problem was indeed the catalytic converter.

Each code scanner is different, but they all show you the standard problem codes and have a function that allows clearing and resetting the computer. I recommend that you read the user's manual. Most good scanners will guide you through the process on the little screen if you carefully follow along with the on-screen prompts.

Check Engine Light most cases

The good news is that it’s not always a disaster when the check engine light comes on. In most cases, when the check engine light is on but solid (not blinking) it may just be an alert that all is not well with your car.

How to fix the solid check engine light?

You can keep driving for a while until you can get to a mechanic. But don’t ignore it or forget to have your car checked as soon as convenient.

Related: Car Warning Lights Resource Centre

Why Gas Caps Fail and What to Check For

1. Gas Cap Is Not Secured

Sometimes a gas cap throws a check engine light because it wasn’t properly secured after being removed. This usually happens at places like a gas station. If you fill up your gas tank and a check engine light comes on after you drive away, it could be from an improperly secured gas cap. To prevent this, always turn the gas cap until it clicks.

2. Broken Gas Cap Seal

Gas cap seals or gaskets help keep vacuum pressure

Gas cap seals or gaskets help keep vacuum pressure inside the gas tank, but when damaged, they can leak pressure. Gas cap gaskets can roll over, fall off, break, crack, and split, all creating opportunity for leaks.

Sometimes the gas cap can break on its own by clicking and looking like it’s snug but never sealing.

3. Incorrect Gas Cap

Not all gas caps are the same and they can vary in

Not all gas caps are the same and they can vary in shape and size. If the wrong one has been installed, it’s not going to secure properly and will throw a check engine light.

5. Spark Plugs

Finally, worn spark plugs or plug wires will result in a check engine warning. Just about anything wrong with your vehicle’s ignition system will likely cause your check engine light to come on. If you haven’t had your plugs changed and your vehicle has over 100,000 miles on it, it’s time for new plugs and possibly new wires. Stalls are another indicator of spark plug trouble.

Stringer Auto Repair, LLC, in Johnstown, OH, offers check engine light diagnostics. Bring your vehicle into our shop or give us a call today.

2. Gas Cap

Hopefully, your check engine warning came on because you didn’t screw on your gas cap tight enough the last time you filled car, SUV, or truck. A loose gas cap will return a check engine warning because there should be a vacuum seal created with the cap once you’ve tightened it. If your cap is loose because you didn’t tighten it, cracked, or if the seal is broken, your check engine warning will come on.

No. 6: Overheating

Chances are, if your check engine light comes on because of overheating, you’ll notice some other signs as well — such as the temperature gauge running high or even smoke coming out from under your hood. Since this could be a severe problem, it should be addressed immediately. Slow down and you may want to turn on the heater as a way to release some of the heat from the engine. If this doesn’t help, you may need to pull over and call for roadside assistance.

How to Fix Check Engine Light

To fix a Check Engine Light that is on, you will f

To fix a Check Engine Light that is on, you will follow a few basic steps. 

1. Buy an OBD Reader

It isn’t expensive to buy your own OBD code reader. With this handy tool, you can check common engine faults without paying a mechanic.

If you don’t want to buy an OBD reader, you can check with your local parts store. Many of the nationwide locations will check the codes for free.

2. Scan for Engine Codes

Under the driver’s side dashboard, you will see a trapezoid-shaped port. That’s where you want to plug in the code reader. Make sure the engine and key are off when you plug it in.

However, once the scanner is hooked up and on, you want to turn the key to your vehicle on but leave the engine off. Press the scan button and watch the codes that come up. 

3. Search the Engine Code

You can take your code to the internet to find the cause of your fault. At Mechanic Base, we keep our readers informed of the most common trouble codes

If you can’t find the information you need online, consider calling your local dealership for help.

4. Replace the Defective Part

Once you know what the problem is, you can purchase the necessary replacement parts and install them. Obviously, some of the repairs are going to cost a lot less than others, such as a replacement gas cap versus a catalytic converter.

However, you should never prolong a repair, as it could lead to more serious issues down the road. 

5. Erase the Code

Once everything is fixed, it’s time to erase the codes. Plug your scanner back in and hold down the erase button. 

You might need to confirm the changes. We recommend rescanning the engine to be sure they are gone. You will also see the Check Engine Light go off.

If you are repairing an issue to pass a smog check, take your vehicle for a few drives before getting retested. Many technicians can tell if you have recently deleted a code, so it’s best to drive for at least 50 miles first.