# Check engine light and the gas cap
**URL:** https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094
**Category:** Factual Questions
**Created:** [October 20, 2016, 2:39am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094 "2016-10-20T02:39:51Z")
**Posts on this page:** 16
**Page:** 1
### Author:  [@turner](https://boards.straightdope.com/u/turner)
#### Post date: [October 20, 2016, 2:39am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/1 "2016-10-20T02:39:51Z")
My daughter’s friend has a new Toyota SUV. The check engine light came on and she took it to the dealer. They told her that she didn’t tighten the gas cap enough. This really frustrated her because that’s one of the problems with her old Toyota that happened so frequently that she just got rid of the car (why she bought another version of the same car? I don’t know). She tightens it as much as she can. I had the same problem intermittently with a Hyundai. Mechanics never report anything else wrong with the car. What else could be causing this and how can she remedy the situation?
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### Author:  [@Jeff\_Lichtman](https://boards.straightdope.com/u/Jeff_Lichtman)
#### Post date: [October 20, 2016, 4:17am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/2 "2016-10-20T04:17:37Z")
On my Toyota you turn the gas cap until it clicks. Does the gas cap on her car work that way?
Is it possible she’s not putting it on correctly? Could she be cross-threading it, or something like that?
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### Author:  [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)
#### Post date: [October 20, 2016, 7:06am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/3 "2016-10-20T07:06:44Z")
My mother had this problem with her Chrysler – after age 80, she couldn’t tighten the gas cape enough, so the ‘check engine’ light kept coming on. If she went to a full service station & the attendant put the cap on, then it was too tight for her to get off when she needed to fill the tank again.
We fixed it by putting a small piece of black electrical tape over the ‘check engine’ light on her dashboard.
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [October 20, 2016, 8:29am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/4 "2016-10-20T08:29:37Z")
What else could be causing it? Theoretically, several different things.
What else is plausibly causing it on a new vehicle that has been checked out and found to have an untightened gas cap and no evidence of other problems? Nothing.
How can she remedy the situation? Keep a pair of pliers in the car large enough to enable her to adequately fasten the cap.
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### Author:  [@bob\_2](https://boards.straightdope.com/u/bob_2)
#### Post date: [October 20, 2016, 10:24am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/5 "2016-10-20T10:24:43Z")
I would think that there is a micro switch on the neck of the tank. It should be possible to find and disable it.
Of course, this would meant that she could drive off without securing the cap, but we used to manage ok before such things were fitted.
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### Author:  [@jz78817](https://boards.straightdope.com/u/jz78817)
#### Post date: [October 20, 2016, 10:52am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/6 "2016-10-20T10:52:46Z")
> [@bob\_2](#):
>
> I would think that there is a micro switch on the neck of the tank. It should be possible to find and disable it.
>
> Of course, this would meant that she could drive off without securing the cap, but we used to manage ok before such things were fitted.
there’s no such switch. cars have to have sealed fuel systems to prevent fuel vapors from escaping the tank (evaporative emissions.) At certain times, the evaporative emissions control system will do a self test, checking via either pressure or vacuum cycling that everything is sealed. a loose gas cap will cause this test to fail. if the cap is tightened sufficiently and it still fails, possible causes could be as simple as a torn gasket on the gas cap, or a crack/rust through on the fuel filler neck, or a crack in the carbon canister.
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### Author:  [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)
#### Post date: [October 20, 2016, 11:02am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/7 "2016-10-20T11:02:32Z")
The “gas cap” error code is actually a pressure test. There’s no micro-switch that is checking that the gas cap is in place. If the system leaks during the pressure test, the “gas cap” error code is thrown. If there’s a crack or a bad seal anywhere in the system, the test will fail. If the pump is bad, the test will fail.
And of course, if you don’t screw the gas cap on properly, the test will fail, or if the rubber seal on the gas cap is bad, the test will fail. These are the most common causes, but not the only causes.
You can poke around on google for your exact model SUV to see if it is prone to particular types of failures that trigger the gas cap code.
ETA: Ninja’d.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [October 20, 2016, 11:55am UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/8 "2016-10-20T11:55:58Z")
> [@jz78817](#):
>
> there’s no such switch. cars have to have sealed fuel systems to prevent fuel vapors from escaping the tank (evaporative emissions.) At certain times, the evaporative emissions control system will do a self test, checking via either pressure or vacuum cycling that everything is sealed. a loose gas cap will cause this test to fail. if the cap is tightened sufficiently and it still fails, possible causes could be as simple as a torn gasket on the gas cap, or a crack/rust through on the fuel filler neck, or a crack in the carbon canister.
This. On my previous car I had a cracked fitting on the canister. It happens.
The gas cap typically has a built in clutch that limits the amount of torque you can apply; this is the mechanism that makes that horrible cracking/clicking sound that sounds like it’s breaking. When you hear that, you know you’ve tightened it enough. If you spin the everloving crap out of it, you may be able to snug it up just a little bit more - but this should not be necessary, and it definitely should not need to be tightened so snugly that a healthy adult female can’t loosen it.
To be sure, she should consult the owner’s manual for the exact instructions on how to tighten the filler cap. For example, here are the instructions for a Honda CR-V, straight from the owner’s manual:
> [@Honda](#):
>
> Screw the fuel fill cap back on until it clicks at least once.
It continues:
> [@Honda](#):
>
> Your vehicle’s onboard diagnostic system will detect a loose or missing fuel fill cap as an evaporative system leak. The first time a leak is detected a ‘‘CHECK FUEL CAP’’ message appears on the information display. Turn the engine off, and confirm the fuel fill cap is installed. If it is, loosen it, then retighten it until it clicks at least once. The message should go off after several days of normal driving once you tighten or replace the fuel fill cap. To scroll to another display, press the select/reset knob.
>
> The ‘‘CHECK FUEL CAP’’ message will appear each time you restart the
> engine until the system turns the message off.
>
> If the system still detects a leak in your vehicle’s evaporative emissions system, the malfunction indicator lamp (MIL) comes on. If the fuel fill cap was not already tightened, turn the engine off, and check or retighten the fuel fill cap until it clicks at least once. The MIL should go off after several days of normal driving once the cap is tightened or replaced. If the MIL does not go off, have your vehicle inspected by a
> dealer.
The instructions for a Toyota should be comparably simple. If she’s not following them, then yeah, the problem is on her end. But if she’s following those instructions and the check-engine light is still indicating an evap system malfunction, then the car needs service to make that light go away, and the problem may not be with the fuel fill cap. If the car is still under warranty (i.e. if it really is _new_), then this will be a warranty issue. If they try to tell her she needs to tighten the cap more than the manual indicates, she should call bullshit on them.
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### Author:  [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)
#### Post date: [October 20, 2016, 1:19pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/9 "2016-10-20T13:19:14Z")
OP, you said the car was new. So the cap and seal (hell, all the seals on the car) are new. I would take it back and ask the dealer look to at the evaporative emissions system.
That being said, this happens every few years on my old Subaru. I just buy a new cap (that has a new seal) and it solves the problem.
Edited to say: If it’s only thrown the code that one time it may, in fact, have been the gas cap.
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### Author:  [@gnoitall](https://boards.straightdope.com/u/gnoitall)
#### Post date: [October 20, 2016, 2:22pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/10 "2016-10-20T14:22:51Z")
> [@somepumpkin](#):
>
> OP, you said the car was new. So the cap and seal (hell, all the seals on the car) are new. I would take it back and ask the dealer look to at the evaporative emissions system.
>
> That being said, this happens every few years on my old Subaru. I just buy a new cap (that has a new seal) and it solves the problem.
>
> Edited to say: If it’s only thrown the code that one time it may, in fact, have been the gas cap.
I’ll also point out that a lot of gas caps can be mis-threaded, just like any coarse-pitch screw thread, and if that happens the cap won’t seal even if you turn to the point of “clicking” resistance.
I say this from personal experience. :o
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### Author:  [@racer72](https://boards.straightdope.com/u/racer72)
#### Post date: [October 20, 2016, 2:54pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/11 "2016-10-20T14:54:15Z")
My wife had the same problem with her 2014 Rav 4. A new non Toyota brand gas cap fixed the problem.
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### Author:  [@Me\_Billy](https://boards.straightdope.com/u/Me_Billy)
#### Post date: [October 20, 2016, 3:19pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/12 "2016-10-20T15:19:51Z")
FYI - Another gas tank related problem is to “top off” the gas tank when filling up. DON’T DO THIS!
That can fill the gas tank ventilation system with liquid gas and block it so it can’t ventilate properly.
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [October 20, 2016, 4:09pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/13 "2016-10-20T16:09:31Z")
> [@racer72](#):
>
> My wife had the same problem with her 2014 Rav 4. A new non Toyota brand gas cap fixed the problem.
I’d say this is certainly worth trying. There are cases where aftermarket parts don’t perform as well as factory ones, but this could be somewhat of a reverse – and aftermarket part that’s easier to use than the factory one.
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### Author:  [@JcWoman](https://boards.straightdope.com/u/JcWoman)
#### Post date: [October 20, 2016, 6:35pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/14 "2016-10-20T18:35:28Z")
This is a tangent, but this blew me away:
> [@](#):
>
> The message should go off after several days of normal driving once you tighten or replace the fuel fill cap.
DAYS of normal driving? WTH? This is even worse than my Prius with it’s generic “one of the tires is low but we’re not going to tell you which one” dummy light that stays on for a while after you inflate all the tires properly. There’s nothing I love more than:
[ul]
[li]start to drive, see the dummy light[/li][li]Go back home and get out my handy air compressor, top off all the tires[/li][li]start to drive, see the dummy light[/li][li]go back home and get out the air compressor… etc until I get fed up and just ignore the stupid light[/li][li]Light “magically” goes off by itself some hours later[/li][/ul]
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [October 20, 2016, 6:52pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/15 "2016-10-20T18:52:01Z")
> [@JcWoman](#):
>
> This is a tangent, but this blew me away:
>
> DAYS of normal driving? WTH? This is even worse than my Prius with it’s generic “one of the tires is low but we’re not going to tell you which one” dummy light that stays on for a while after you inflate all the tires properly. There’s nothing I love more than:
>
> [ul]
> [li]start to drive, see the dummy light[/li][li]Go back home and get out my handy air compressor, top off all the tires[/li][li]start to drive, see the dummy light[/li][li]go back home and get out the air compressor… etc until I get fed up and just ignore the stupid light[/li][li]Light “magically” goes off by itself some hours later[/li][/ul]
The “days of normal driving” means X number of drive cycles without the particular condition recurring.
As to your tire pressure monitor, some systems require a certain amount of driving time with acceptable readings before they turn the light off. I agree it seems [del]silly[/del] stupid. I’d say a good strategy is to ensure the tire pressure is okay with your pressure gauge, then ignore the light for a fair while. Let the light go nuts waiting for you to acknowledge it rather than you go nuts trying to appease the light. 🙂
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### Author:  [@cochrane](https://boards.straightdope.com/u/cochrane)
#### Post date: [October 20, 2016, 7:35pm UTC](https://boards.straightdope.com/t/check-engine-light-and-the-gas-cap/769094/16 "2016-10-20T19:35:28Z")
> [@JcWoman](#):
>
> This is a tangent, but this blew me away:
>
> DAYS of normal driving? WTH? This is even worse than my Prius with it’s generic “one of the tires is low but we’re not going to tell you which one” dummy light that stays on for a while after you inflate all the tires properly. There’s nothing I love more than:
>
> [ul]
> [li]start to drive, see the dummy light[/li][li]Go back home and get out my handy air compressor, top off all the tires[/li][li]start to drive, see the dummy light[/li][li]go back home and get out the air compressor… etc until I get fed up and just ignore the stupid light[/li][li]Light “magically” goes off by itself some hours later[/li][/ul]
The owner’s manual for my car is more specific. After I loosen the gas cap and re-tighten it, I should start it, drive and let the engine warm up to normal temperature, shut it off, and let it get cold. I should do this three times. This can be over the course of normal driving, running errands, etc. I don’t have do it three times in a row just to get the engine light to go off. But it does have to be long enough to get the engine up to normal operating temperature.