There are many reasons you see that little monster—check engine light after getting gas. It could be a damaged seal on the gas cap, a loose gas cap, a broken vacuum hose, or bad fuel.
Newer model cars, especially those built after the 90s, have evaporative emission control (EVAP) systems that collect fuel vapor and prevent it from getting into the atmosphere. This system is designed to keep the environment clean.
Essentially, when the fuel evaporates inside your gas tank, the EVAP system will store the harmful vapor in a separate charcoal canister. When the fuel-to-air mixture becomes normal, it’ll be transferred back into the intake manifold. The engine control unit (ECU) constantly monitors this pressurized and sealed system while the engine is running. If the ECU detects pressure loss, it’ll trigger the check engine light on the dashboard, showing there’s an underlying issue. It could be a loose gas cap, bad fuel, or something else.
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Why check the engine light after getting gas?
Why is my check engine light on after getting gas? There are a couple of simple reasons your check engine light came up after getting gas. A loose gas cap, an overfilled tank, a damaged vacuum hose, and a lousy gas cap seal are the common reasons the car computer throws the check engine light on the dashboard.
Loose gas cap
A moment of haste at the gas station can lead to an unwelcome surprise: the illumination of the check engine light. This usually happens when you forget to tighten your gas cap properly. The fuel systems in modern cars are designed to prevent fuel vapor from getting into the atmosphere. But if you do not tighten the cap properly, it’ll allow fuel vapor to constantly escape into the atmosphere.
Once you turn on the vehicle and your car computer notices this anomaly, it will trigger the check engine light on the dashboard to let you know something is wrong with the system.
Worn-out gas cap seals
The gas cap has a seal that ensures it is air-tight when you cover it. This seal can, however, degrade over time due to age. When that happens, it will lose its elasticity and ability to prevent fuel vapor from escaping the fuel system, and the engine control unit will think there is a fuel leak and display the engine warning light so you can take necessary actions.
Using bad fuel
Trust me; it could be you just filled in contaminated or low-grade fuel into your gas tank. If your car takes longer to start after refueling, rough idle, or misfires right from the filling station, there’s a high chance you just bought contaminated fuel or regular instead of premium gas.
Contaminated and incorrect fuel grades will make your car act funny. Once the powertrain control module (PCM) or the car computer detects the contamination, the check engine light will be triggered.
You left the engine running while refueling the tank
Did you leave the engine running when refueling the car? If yes, the car sensors may be operational when you were refueling and may have detected a pressure loss within the fuel system. When these sensors detect a pressure loss, they will signal the PCM/ECM, which will then display the engine warning light on the dashboard.
Overfilled gas tank
You may be asking, can overfilling gas tank cause check engine light? If you overfill your gas tank, the liquid fuel may cover the vapor intake hole, so instead of the charcoal canister to suck fuel vapor, it will suck in liquid fuel. This, however, can damage the canister and, possibly, other system components, which will cause the check engine light to appear.
What to do to clear the check engine light
The most common fix is to make sure your gas cap is airtight, the gas tank is not overfilled, and you turn off the engine while refueling your tank.
Do not drive off right away until you tighten the gas cap properly. Sometimes, the warning light will not disappear until you run several cycles. How long before check engine light goes off after gas cap? You could drive 3-5 days before the light goes away.
If the warning light persists, diagnose the vehicle and clear it with a scan tool. If you diagnose the vehicle, you will likely see logged error codes P1440 and P0442, showing there’s an issue with the evaporative emission control system.
Don’t worry if this light stays on after retightening the cap. You can still drive your car for some days before experiencing any damage. That, however, does not mean you should ignore the light.
Why you shouldn’t ignore a check engine light
Regardless of why the check engine light is on, do not ignore it. When you see the check engine light on the dashboard, your car computer telling you that something has failed and will ultimately cause damages to your vehicle, which will be expensive to repair.
Essentially, the car computer may trigger the engine warning light again when it detects more severe issues, but since the light is already on, you won’t know it has been triggered for a serious issue. This will invariably cost you more to fix.
What are the consequences of ignoring a check engine light?
If your check engine light appears after refueling, it’s okay to leave it for a few drive cycles. The light will disappear, especially if it came on because you left your engine running while refueling at a gas station. But if the light persists for long, diagnose the vehicle and clear it with a scan tool.
Ignoring the check engine light will cause the problem that triggered it in the first place to get worse and affect your car transmission or engine. Also, ignoring the light could lead to more long-term problems that are not covered by your insurance.
You won’t pass your car emission test with the check engine light on. You must fix the issue and wipe the check engine light before you can pass the smog inspection.
Final words
Check engine light after getting gas could mean you have a loose gas cap, the gas attendant overfilled the gas tank, you left the engine running while refueling your tank, or you have a lousy gas cap seal. Retightening the gas cap or simply driving a few cycles to let the system reset could be all you need to resolve the issue.
To sum it up, check engine light doesn’t necessarily mean you have a serious problem. However, if you let the issue linger for long, serious problems may arise. If you retight the gas cap, drive a few cycles, and yet the light persists, have a mechanic diagnose the car and address the issue.
