Home Fuel System Gas Cap Check Engine Light – As Simple Of A Fix As It Seems?

Gas Cap Check Engine Light – As Simple Of A Fix As It Seems?

by Kelvin Yates

Of all the myriad of warning lights that a car can throw at you, the check engine light is undoubtedly the most concerning one. More often than not, it’s a sign that something is seriously amiss with your car. For instance, an issue with the emissions systems, faulty sensors and electronics, or concerns with the engine. All of which can cost you thousands to repair… Except for the gas cap check engine light.

Yep, you heard that right. While the check engine light (or CEL) is frequently associated with pricey or complicated repairs, it can also trigger due to less pressing problems. Such as, when your gas cap is a bit loose or has gone missing. But if it’s merely a plastic cap, then why light up such a heart-sinking warning light? Well, it turns out that maybe, a gas cap check engine light can be quite problematic.

What Does Your Gas Cap Do, And Why Trigger The Check Engine Light?

As we’re trying to wrap our heads around a gas cap check engine light, it’s worthwhile answering the question of what does it even do? More precisely, what function does your gas cap serve that having it being broken or missing is worth springing up a check engine light for? All it serves is as a plastic lid to keep the gas out, right? Well, this is true in older cars, as a way to prevent fuel from spilling out.

However, this simplistic functionality is no longer the case in more modern cars for the past couple of decades. In today’s cars, the gas cap serves numerous functions. Even so, there are cars out there that don’t even have a twistable gas cap. Instead, they use a self-latching seal door. Either way, they both are designed to maintain a complete and tight seal around the tank, keeping fuel vapors in.

Gas Cap Check Engine Light

In addition, they’re no longer a simple cap over the fuel filler neck of your gas tank. In part due to increased pressure to control vehicular emissions, the gas cap (as we’ll learn more later) is a key part of your car’s emissions control. Moreover, that unseemly plastic lid is integrated into most cars’ OBDII onboard diagnostics systems. Hence, allowing your car to monitor the condition of your fuel.

Throughout most states, an I/M240 emissions test involves making sure the gas cap works. A simple method for you to know if it’s seated properly is by hearing that solid “click” when you turn it. While we’re here, it’s worth noting that gas caps aren’t universal. Not all of them are built according to the automakers’ specs, while some don’t adhere to the state-mandated I/M240 emissions test.

Functions Of A Gas Cap, And Why A Check Engine Light Matters

So, why is it that important? Well, there are a few jobs that your car’s gas cap needs filling:

  • Safety Concerns – In keeping fuel neatly sealed within the tank, there’s a significantly decreased risk of it spilling out. Thus, lowering the chance that a fire may engulf your car in the event of an accident, or any collision that may compromise the structural integrity of the tank. Remember that gasoline is an incredibly volatile liquid that can vaporize quite readily, even at room temperature.
  • Better For The Environment – Fuel vapors are a major cause of ground-level ozone pollution. Starting in the early 1970s, all US-registered cars needed some evaporative system built into the fuel system to control fuel vapors. The gas cap sealed all fuel vapors inside the tank, coupled with a charcoal canister to subsequently cut down on fuel vapor leakage and atmospheric emissions.

  • Fuel Economy – Keeping vapor sealed tightly within can also contribute to better fuel economy. As we noted earlier, gasoline evaporates easily, even at room temperature. As such, a large quantity of gasoline in the tank typically exists as a vapor. If that were to escape, there’s less fuel for you to use. Statistically, a missing or poorly installed cap can lose as much as 30 gallons of gas every year.
  • Contamination – Another crucial task undertaken by your gas cap is optimizing performance. This can be accomplished by preventing outside contaminants from breaching into the gasoline supply. Water, for example, could cause serious damage to your car’s fuel system. Meanwhile, dirt, debris, and other impurities in the fuel can severely impact performance and longevity once they get into the engine.

How To Diagnose Gas Cap Check Engine Light Issues?

So then, if your check engine light could be triggered by any number of issues, how can you be certain that the gas cap is the cause? For you to be sure, a thorough diagnosis of gas cap check engine light issues is necessary to determine the suspect. When that’s done, you’re all set for repairs. Here are a couple of ways that you can quickly diagnose gas cap check engine light, and see what’s wrong:

1. Physical Inspection – Check If The Gas Cap Is Loose, Broken, Or Missing

Once you spot the check engine light appearing, either as a static light or rapid flashing, the first thing you should check is the gas cap. Turn off the ignition, pop open the filler door, and inspect the gas cap in detail. It’ll be quite obvious if the gas cap is missing. Perhaps you left it behind at the pump? Other than that, you’ll have to look closely at any damage or looseness in the cap. In particular:

Gas Cap Check Engine Light

  1. Remove the gas cap, and note down if there are any physical cracks, chips, or tears in the plastic gas cap lid.
  2. Check the seal between the gas cap and fuel filler tube, and see if there are any tears or cracks on that seal.
  3. Install the gas cap on the filler tube, and try to tighten it in place. You should hear a solid “click” once it’s sealed shut. If you don’t hear that “click”, then the cap might be faulty.
  4. Alternatively, if you tighten the gas cap until that “click” is heard, wiggle it around to notice any play or looseness once it’s seated. Normally, it should be held firmly in place.

Therefore, if your gas cap has borne physical damage, doesn’t click into place, or has significant play and looseness, then you’ll know that it needs replacing. If the check engine light was caused by that gas cap, then it should go off after a few minutes of driving. However, it might keep lighting up and turning off repeatedly, warning you to pay close attention to the loose or damaged gas cap.

2. OBDII Diagnostics Tool – Scan Your Car For Gas Cap-Related Error Codes

Remember when we talked about how modern-day gas caps are intertwined with OBDII diagnostics? This has meant that diagnosing your gas cap can be as easy as plugging in an OBDII scanner and then reading into what error codes it spews out. These diagnostic trouble codes (or DTC) can point towards exactly what’s prompting the check engine light to appear in the first place, including the gas cap.

You can find an abundance of OBDII scanners on Amazon for mere tens of dollars. Most of which are easily compatible with a wide variety of cars that support the OBDII standard. Some of them display the error codes and what they’re defined as clearly. Note, that cheaper OBDII scanners only show the codes, which forces you to check online for what they mean. This should still be sufficient enough.

Now, plug the OBDII scanner into the OBDII port (usually found under the steering column behind a plastic cover or door), and let it scan. When it’s completed scanning, it’ll show you all the codes that came up. In particular, below are a list of the codes that may relate to a gas cap check engine light. It should be warned that not all of these codes may immediately warn you of gas cap-related problems.

Some of them might point to other faults that relate to your car’s emissions control. Nevertheless, it does at least help to narrow down the search. Your car runs regular EVAP self-tests to make sure that there are no fuel vapor leaks to trigger a gas cap check engine light. When the fuel system is unable to withstand the pressure (in other words, a leak), the gas cap check engine light comes up.

2.1. Gas Cap Check Engine Light OBDII Trouble Codes:

  • P0440 – Evaporative Emission Control System Malfunction
  • P0441 – Evaporative Emission Control System Incorrect Purge Flow
  • P0442 – Evaporative Emission Control System Leak Detected (Small Leak)
  • P0443 – Evaporative Emission Control System Purge Control Valve Circuit Malfunction
  • P0444 – Evaporative Emission Control System Purge Control Valve Circuit Open
  • P0445 – Evaporative Emission Control System Purge Control Valve Circuit Shorted
  • P0446 – Evaporative Emission Control System Vent Control Circuit Malfunction
  • P0447 – Evaporative Emission Control System Vent Control Circuit Open
  • P0448 – Evaporative Emission Control System Vent Control Circuit Shorted
  • P0449 – Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction
  • P0452 – Evaporative Emission Control System Pressure Sensor Low Input
  • P0453 – Evaporative Emission Control System Pressure Sensor High Input
  • P0455 – System Gross Leak Evaporative Emission
  • P0456 – Evaporative Emissions Control System Small Leak Detected
  • P0457 – Evaporative Emission Control System Leak Detected

To be more specific, these three diagnostic trouble codes (among the abovementioned ones) typically point directly to a malfunctioning gas cap:

  • P0455 (System Gross Leak Evaporative Emission) – There are large and substantial fuel vapor leaks in the fuel system. Although, the gas cap might not be the only culprit, as there could be leaks elsewhere if the leakage is too significant.
  • P0456 (Evaporative Emissions Control System Small Leak Detected) – As the definition itself implies, it triggers in the event that smaller gas vapor leaks are detected. We can commonly find the cause of the fault to lie somewhere in the gas cap, fuel filler, or seal in-between.
  • P0457 (Evaporative Emission Control System Leak Detected) – It also denotes smaller leaks, but ones that aren’t consistently leaking. We can usually blame a loose or missing gas cap for this one, as the leakage is mostly on and off. Or, a relatively insignificant volume is leaked into the atmosphere.

What Other Symptoms Are There Besides Gas Cap Check Engine Light?

Now, you might be wondering as to why a gas cap check engine light needs to appear, anyway. If this component is as consequential to a car’s performance, fuel economy, and emissions as it seems, then how and why can it fail? Simply put, the gas cap undergoes plenty of use anytime you’re filling up. It’s going through endless cycles of being screwed down and unscrewed at least once during a pit stop.

The intense wear and tear are only exacerbated when you consider that some folks might fill up more than once in a week. Over its lifetime, this frequent usage will wear down the seal on the gas cap. It might be difficult for you to spot when the seal has worn all the way down, besides seeing a gas cap check engine light. Still, there are other ways that you can ascertain if your gas cap is at fault.

Gas Cap Check Engine Light

Besides a gas cap check engine light, here are a few other noteworthy symptoms of a broken gas cap aside from seeing a warning light:

1. Lack Of A Hissing Noise When You Unscrew The Gas Cap

Have you ever noticed how unscrewing the gas cap would emit a hissing sound? This isn’t an issue, as this hissing noise is an indicator that your gas cap is working as it should. It hisses because, as you’re unscrewing the cap, the vacuum seal between the gas cap and fuel filler neck is opened up.

Thus, outside air rushes into the tank, emitting a noticeable hiss or whoosh. You can also use this hiss as a way to know if your gas cap is on its way out. Should it emit a less distinct hissing sound, or none at all, it likely means that the vacuum seal formed by the gas cap is compromised, leaking out vapor.

2. Gas Cap Doesn’t Tighten As Much As It Normally Should

Remember that “click” sound we mentioned earlier? It’s there by choice, for safety reasons, as a way to let you know when the gas cap is sufficiently tightened. If you screw the gas cap in and don’t hear that solid “click” by the end of twisting it in, that should be a clear-cut symptom that it’s faulty.

Or, and as we highlighted earlier in the diagnosis, be mindful of any visible looseness or play in the lid when it’s supposed to be tight. A gas cap should seat firmly in place, without any significant wiggling around. You can then inspect and diagnose to spot any physical damage on the gas cap.

3. Poor Fitment And Seating Of The Gas Cap

It’s somewhat related to our previous point, but the fitment of the gas cap should be firm and tightly in place on the end of the fuel filler. This problem is common with older vehicles with metal gas caps, as heat or wear can deform them more easily. In contrast, modern cars have plastic gas caps.

Nonetheless, modern cars have to contend with rubber seals to maintain that air-tight vacuum seal. As they age, the rubber seals can harden, crack, or turn brittle. Additionally, the integrity of the seals can be prematurely worn out by dirt or dried-out fuel spilled onto it. It’s worse if that fuel is diesel.

We also have to point out, once more, that not all gas caps are interchangeable. Some might claim to be “universal”, for example. While they might be able to fit most cars from a large conglomerate, it’ll fail to accommodate every single vehicle. So, be mindful when you’re shopping for a new one.

4. Faint Or Strong Scent Of Fuel Outside And Inside The Car

With a damaged or loose gas cap that can’t create an adequately strong seal, fuel vapors will start escaping into the air. Your nose should pick up this scent right away, although it might be fairly faint at first. Most often, this distinct gasoline smell will only be prevalent outside of the vehicle.

That said, the scent can, if not fixed and sealed in time, permeate into the cabin. At this point, it can become a serious health hazard. Directly inhaling excessive fuel vapor can eventually lead to carbon monoxide poisoning. Gradually, it’ll leave permanent damage to your lungs.

5. Discoloration Around The Fuel Filler Opening

Since a bad gas cap will lead it to leak out fuel vapor, it’ll eventually have to go through the fuel filler opening. This poor seal would enable gasoline (or diesel) fumes to leave deposits around the gas tank opening as a parting gift. You can notice this with odd coloration or discoloration around it.

Look around, and see if you can spot discoloration on the plastic paneling on the fuel filler, gas cap, as well as the inside of the fuel door. Some cars might have the inner side of the fuel door painted in body color. So, see if the shade of paint has faded or looked worn compared to the rest of your car.

How Much Is For A Gas Cap Replacement Cost?

After discussing in great length about all you need to know that precedes a gas cap check engine light so far, how can you go about repairing it? Quite simply, a gas cap isn’t a repairable item. Sure, there are certain cases where the rubber seal could somehow be re-sealed. But what inconvenience you pay isn’t worth simply replacing the gas cap outright for a new one. It doesn’t cost that much, either.

Just log into Amazon, and you can find an ample supply of gas caps for around $10 to $20. Although, remember to get the right gas cap that matches your vehicle’s make, model, and model year. While you’re at it, always get OEM-specific gas caps, and avoid those so-called “universal” type ones if you can. Moreover, try not to opt for no-brand gas caps that don’t specify what vehicle it’s made for.

Gas caps all require a precise fitment to ensure a tight seal, one which you can’t compromise on. But once you’ve already installed the new gas cap, your car might still throw up a gas cap check engine light at you. The next step in the process of mending that gas cap check engine light error is to clear out and reset your car’s computer. This should prevent it from lighting up again.

To get your car to recognize the new gas cap and clear out that gas cap check engine light alert, here are a few steps of where to get started:

How To Fix, And Reset A Gas Cap Check Engine Light?

  1. First, turn off your car’s engine, and unlock the fuel filler door. This latch is usually on the floorboard by the driver’s side seating, or under the steering column.
  2. Next, pop open the fuel filler door and unscrew the gas cap by twisting it counter-clockwise. Remove the old gas cap, and grab the replacement unit.
  3. Now, turn the new gas cap clockwise to screw it tightly in place, until you hear that solid “click”. Just to be absolutely sure that it’s solidly in place, keep screwing it until you hear 3 clicks.
  4. Close the fuel filler door, and crank the engine. Note, that the gas cap check engine light or loose cap warning might not disappear right away. You’ll likely have to drive it around for a little while, usually around a day, before it turns off for good.
  5. Should the gas cap check engine light not go away at all, then you probably have to reset it manually. If that’s the case, connect an OBDII diagnostics tool to your car. Within the user interface, there ought to be a Reset function that’ll forcefully clear all error codes and reset the system. Similarly, hop into the car after clearing out the error codes, and keep driving for the new gas cap to be recognized.

Check Engine Light and Gas Cap Relationship – Facts

  1. The check engine light turning on in your car can be caused by several factors, including a loose or faulty gas cap, a worn spark plug or wire, a faulty oxygen sensor, catalytic converter trouble, and a faulty mass airflow sensor.
  2. If your car’s check engine light comes on, you should pull the car over to the shoulder as soon as it is safe to do so, especially if your car is smoking or making an unusual noise.
  3. If the check engine light comes on and your car is not smoking or making an unusual noise, you can check the gas cap to see if it’s loose or faulty, which may be causing the problem.
  4. To check if your gas cap is loose, open the gas cap door, inspect the cap for any cracks or tears, and then fully install it into the filler tube and tighten it by hand until you hear it click in place.
  5. If the gas cap is not damaged, but it’s still loose after clicking into place, it needs to be replaced.
  6. If the check engine light was caused by a loose gas cap, it should go off after you drive for several minutes.
  7. If the light keeps coming on, and goes off again once you tighten the gas cap, then your gas cap is too loose.
  8. Ordering a replacement gas cap is simple, but you must make sure it’s suitable for your vehicle’s make and model to ensure a tight fit.
  9. Addressing a loose or faulty gas cap promptly can save you time and money, as it is a relatively minor problem to fix.
  10. Knowing the common causes of a check engine light, such as a loose gas cap, can help you troubleshoot the problem and alleviate any fears you may have about driving your car when the light turns on.

Final Thoughts On Gas Cap Check Engine Light

That then is a good place to round-up our guide here on gas cap check engine lights. As we can see, it doesn’t take much to trigger the check engine light. After all, this singular warning light on your dash has to warn you of a myriad of issues altogether. The problems could turn out to be multi-thousand dollar repairs. Or, it might require a simple $10 gas cap replacement at your local parts store.

And what a bargain it is given the importance of this plastic lid concerning your car. It might seem like a throwaway item, but there will be dire consequences when it does fail. Your car will end up plagued with poor performance, increased emissions, and the smell of fuel. Plus, you’ll soon realize that you’re spending a whole lot more on fill-ups. As such, do pay close attention to this unassuming plastic lid.

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