# Stupid Check Engine Light
**URL:** https://boards.straightdope.com/t/stupid-check-engine-light/709870
**Category:** In My Humble Opinion
**Created:** [January 15, 2015, 3:06am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870 "2015-01-15T03:06:09Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@gigi](https://boards.straightdope.com/u/gigi)
#### Post date: [January 15, 2015, 3:06am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/1 "2015-01-15T03:06:09Z")
OK, I have a 2008 Corolla and the Check Engine light has come on at least four times, and the verdict every other time has been emissions, aka the gas cap. I promise I close it the same way every time, but whatever. Anyway, at least twice, by the time I took it in, the light had gone out. In an effort to understand this wily beast:
1. Does the car somehow periodically check for emissions, and that’s why it will have the light on and then a couple of days later it goes out? Is that also why the light doesn’t come on right away after I fill up, but a few days later? I do jiggle the gas cap at some point in there but does that really do anything?
2. Will it help to get a new gas cap and if so, does it make a difference if it is a genuine Toyota part or not?
I do know to PULL OVER IMMEDIATELY if the light starts flashing but other than that I am trying to learn to live with this thing, if it’s going to do this so often. (OK, maybe it’s not a lot but it came on maybe once in ten years in my Civic.)
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### Author:  [@carnut](https://boards.straightdope.com/u/carnut)
#### Post date: [January 15, 2015, 3:19am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/2 "2015-01-15T03:19:15Z")
gigi, I used to have that issue with my VW. You may actually need a new catalytic converter. Even the dealer told me it was the gas cap, but once I splurged on a replacement catalytic converter (something we all should keep current on or stop breathing air) the problem never appeared again.
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### Author:  [@jz78817](https://boards.straightdope.com/u/jz78817)
#### Post date: [January 15, 2015, 4:21am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/3 "2015-01-15T04:21:58Z")
> [@gigi](#):
>
> OK, I have a 2008 Corolla and the Check Engine light has come on at least four times, and the verdict every other time has been emissions, aka the gas cap.
um, no. the gas cap is not the only “emissions” thing. The check engine light may come on for _any_ fault in the powertrain management system that can cause the vehicles emissions (either evaporative or out the tailpipe) to exceed what the car was certified for.
> [@](#):
>
> 1. Does the car somehow periodically check for emissions, and that’s why it will have the light on and then a couple of days later it goes out? Is that also why the light doesn’t come on right away after I fill up, but a few days later? I do jiggle the gas cap at some point in there but does that really do anything?
The car is always monitoring itself for potential faults. A loose gas cap (or one with a bad gasket) will cause an evaporative emissions fault, meaning the loose cap is letting raw fuel vapors escape the tank. Every time you turn the car on, the engine tries to draw a partial vacuum in the fuel system (tank/filler neck/vapor canister.) if it can’t draw enough of a vacuum, it’ll set an evap fault and put the check engine light on. this is what typically happens with a loose gas cap. but a loose gas cap is far from the only thing that can cause a check engine light.
> [@](#):
>
> 1. Will it help to get a new gas cap and if so, does it make a difference if it is a genuine Toyota part or not?
go ahead and get a new gas cap if you want. but don’t be surprised if it doesn’t fix anything. There are hundreds of potential problems which can cause the check engine light to come on. Here’s a nice long list:
> **[OBD-II Check Engine Light Trouble Codes](https://www.obd-codes.com/trouble_codes/)**
>
> OBD-II OBD2 Diagnostic Trouble Codes. Definitions and suggested repair procedures
only one or two of those might be caused by a loose gas cap.
> [@](#):
>
> I do know to PULL OVER IMMEDIATELY if the light starts flashing but other than that I am trying to learn to live with this thing, if it’s going to do this so often. (OK, maybe it’s not a lot but it came on maybe once in ten years in my Civic.)
Take it to someone who can read the codes from the PCM and see what they say. Until you know what trouble codes have been set, NOBODY can give you advice on what to do next.
> [@carnut](#):
>
> gigi, I used to have that issue with my VW. You may actually need a new catalytic converter. Even the dealer told me it was the gas cap, but once I splurged on a replacement catalytic converter (something we all should keep current on or stop breathing air) the problem never appeared again.
Stop giving bad advice.
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### Author:  [@Joey\_P](https://boards.straightdope.com/u/Joey_P)
#### Post date: [January 15, 2015, 4:38am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/4 "2015-01-15T04:38:39Z")
> [@jz78817](#):
>
> Every time you turn the car on, the engine tries to draw a partial vacuum in the fuel system (tank/filler neck/vapor canister.) if it can’t draw enough of a vacuum, it’ll set an evap fault and put the check engine light on.
It’s my understanding that the fuel tank is actually slightly pressurized and if the computer noticed that the pressure isn’t correct it’ll turn on the CEL. It’s most commonly the gas cap, but it could be something else. As for why it take a few days…many fault codes take a certain amount of days, miles, warm up/cool down cycles or no fault starts with the code not be thrown before the computer will turn off the light. The gas cap is one of them. So if you get gas and don’t tighten the cap all the way and the CEL comes on, then you fix it, it can take a day or two to turn back off.
It could also just be a bad gas cap. You could try getting a new one and see if that fixes it. Very quickly looking online, I see generic ones for about $6 and OEM ones for about $20. Personally, I’d probably get the OEM one just so if I still have problems I can eliminate that as one of the problems. Also, if the price were close (say, less than $30), I’d probably get it from the dealership rather than Amazon. Again, so if I do have to take it in to get looked at and they suggest it’s a bad gas cap I can tell them it was just replaced with an OEM one from them and they can move on to the next thing without wasting any of their time and my money.
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### Author:  [@usedtobe](https://boards.straightdope.com/u/usedtobe)
#### Post date: [January 15, 2015, 5:53am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/6 "2015-01-15T05:53:12Z")
You can buy a code reading device for $60-$100 - you find the trapezoid-shaped connector (usually under the dash near the steering wheel (some get clever and hide it behind the ashtray).
Follow directions and get the code (I bought one with the English description database).
Now you know what causes the light.
In my case, it was the IAT sensor - Intake Air Temperature - it controlled the mixture.
In CA Auto Zone used to offer free readings (they make their money selling the indicated parts). As mechanics were charging $80 to plug in a box, they whined to the State, which sent Auto Zone a nastygram telling them to stop the freebies.
I wouldn’t be surprised if they were back to doing the readings - look around.
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### Author:  [@Doctor\_Jackson](https://boards.straightdope.com/u/Doctor_Jackson)
#### Post date: [January 15, 2015, 3:39pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/7 "2015-01-15T15:39:44Z")
> [@usedtobe](#):
>
> You can buy a code reading device for $60-$100 - you find the trapezoid-shaped connector (usually under the dash near the steering wheel (some get clever and hide it behind the ashtray).
> Follow directions and get the code (I bought one with the English description database).
> **Now you know what causes the light**.
[bolding mine] No, all you know is the code(s) stored in the On Board Diagnostic Computer (ODBC). Many of the codes are very generic and could be caused by a number of different parts. The OP doesn’t seem to me to have the knowledge/experience to interpret the codes and fix them.
A better bet in this case, IMHO, would be to take the vehicle to a good mechanic who can both obtain and interpret the code(s). Once the code(s) are known, further testing is often needed to pinpoint the underlying issue(s).
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### Author:  [@robert\_columbia](https://boards.straightdope.com/u/robert_columbia)
#### Post date: [January 15, 2015, 4:01pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/8 "2015-01-15T16:01:46Z")
I thought the Check Engine light was a warning from your car that it had detected that your mechanic had repaired your car with knockoff parts from the former Soviet bloc.
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### Author:  [@gigi](https://boards.straightdope.com/u/gigi)
#### Post date: [January 15, 2015, 4:07pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/9 "2015-01-15T16:07:45Z")
> [@Doctor\_Jackson](#):
>
> A better bet in this case, IMHO, would be to take the vehicle to a good mechanic who can both obtain and interpret the code(s). Once the code(s) are known, further testing is often needed to pinpoint the underlying issue(s).
At least once I took it to the Toyota dealership and they interpreted the code to probably be the gas cap. A couple of other times I took it to the neighborhood mechanic and they interpreted the code to probably be the gas cap. How far do I push this?
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### Author:  [@Joey\_P](https://boards.straightdope.com/u/Joey_P)
#### Post date: [January 15, 2015, 4:36pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/10 "2015-01-15T16:36:11Z")
> [@gigi](#):
>
> At least once I took it to the Toyota dealership and they interpreted the code to probably be the gas cap. A couple of other times I took it to the neighborhood mechanic and they interpreted the code to probably be the gas cap. How far do I push this?
So, several times people have said the code was probably the gas cap. I’d start by getting a new gas cap. It about the cheapest thing you could do to troubleshoot this problem. Even an OEM one from the dealer isn’t that expensive and there’s no labor involved (unless you have the dealership people connect the little tether at $90/hr).
I’m not sure what you mean by “how far do I push this”, just get a new gas cap and see what happens. If the light keeps coming on, at least you can tell the mechanic/service writer that you’ve already replaced the gas cap so it’s probably not that.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [January 15, 2015, 4:43pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/11 "2015-01-15T16:43:58Z")
> [@gigi](#):
>
> At least once I took it to the Toyota dealership and they interpreted the code to probably be the gas cap. A couple of other times I took it to the neighborhood mechanic and they interpreted the code to probably be the gas cap. How far do I push this?
The code they’re reading just points to [a general leak in the evaporative emissions recovery system.](http://nissanhelp.com/diy/obd_codes/p0442_nissan.html) As you can see at that (Nissan) example link, there are many possible causes. If all you’re paying them for is to read the code, then that’s all they can say with certainty. The cheapest/easiest thing to **try** is replacing the gas cap; it just takes a couple of bucks and a couple of minutes, so they’ll also be happy to suggest that, but with no guarantee of success, since it could be something else in the system (and you haven’t paid them to figure out what).
I had an evap code on my previous car, and the problem in my case was a cracked fitting on the evap cannister. I’d recommend taking the car back to the dealership and paying them for a half-hour of diagnostic time so that instead of telling you “it’s probably the gas cap,” they can tell you “it’s definitely X” where X is some other component in the whole EERS. Then you can decide whether you want to replace that component yourself, or pay them to do it for you.
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### Author:  [@SmellMyWort](https://boards.straightdope.com/u/SmellMyWort)
#### Post date: [January 15, 2015, 5:32pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/12 "2015-01-15T17:32:50Z")
> [@usedtobe](#):
>
> You can buy a code reading device for $60-$100 - you find the trapezoid-shaped connector (usually under the dash near the steering wheel (some get clever and hide it behind the ashtray).
> Follow directions and get the code (I bought one with the English description database).
> Now you know what causes the light.
> In my case, it was the IAT sensor - Intake Air Temperature - it controlled the mixture.
>
> In CA Auto Zone used to offer free readings (they make their money selling the indicated parts). As mechanics were charging $80 to plug in a box, they whined to the State, which sent Auto Zone a nastygram telling them to stop the freebies.
>
> I wouldn’t be surprised if they were back to doing the readings - look around.
Or [$20](http://www.amazon.com/Autel-MaxiScan-MS300-Diagnostic-Vehicles/dp/B001LHVOVK/ref=sr_1_1?s=automotive&ie=UTF8&qid=1421342155&sr=1-1&keywords=obd+ii+scanner&pebp=1421342160700&peasin=B001LHVOVK). I’d search out any Corolla owner forums online and see if anyone else has had similar problems.
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### Author:  [@gigi](https://boards.straightdope.com/u/gigi)
#### Post date: [January 15, 2015, 5:33pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/13 "2015-01-15T17:33:22Z")
Thanks, I will try the new gas cap first.
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [January 15, 2015, 5:37pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/14 "2015-01-15T17:37:05Z")
Apparently the car has a code that could be caused by a faulty gas cap, among other things. For most such codes, a bad cap is the most common cause. As it’s cheaper to replace the cap than to test it, this is a case where testing via parts replacement makes sense. Put a new cap on and see whether or not the code resets.
For many cars, field experience has shown that aftermarket caps don’t always work as well as the OEM (dealer) ones. Getting a Toyota brand cap for your car eliminates this concern, and is what I recommend.
ETA: Whoops! I should have previewed, as I see you’ve already decided to replace the cap.
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### Author:  [@gigi](https://boards.straightdope.com/u/gigi)
#### Post date: [January 15, 2015, 5:44pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/15 "2015-01-15T17:44:35Z")
Thanks, **GaryT**! I got it through amazon but it says Genuine Toyota so hopefully that’s legit.
> **[Amazon.com: Toyota Fuel Cap - 77300-06040 : Automotive](https://www.amazon.com/Genuine-Toyota-77300-06040-Fuel-Assembly/dp/B005BU5DCO)**
>
> Buy Toyota Fuel Cap - 77300-06040: Fuel Tank Caps - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
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### Author:  [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)
#### Post date: [January 15, 2015, 7:17pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/16 "2015-01-15T19:17:31Z")
> [@usedtobe](#):
>
> In CA Auto Zone used to offer free readings (they make their money selling the indicated parts). As mechanics were charging $80 to plug in a box, they whined to the State, which sent Auto Zone a nastygram telling them to stop the freebies.
IANAL but I don’t see how the State could **possibly** stop Auto Zone from doing this, they are not violating any law, and any law passed saying that they are would be unconstitutional IMO.
So yes, just take it to Auto Zone to have the code read.
> [@gigi](#):
>
> I do know to PULL OVER IMMEDIATELY if the light starts flashing but other than that I am trying to learn to live with this thing, if it’s going to do this so often. (OK, maybe it’s not a lot but it came on maybe once in ten years in my Civic.)
I’m a car guy and I have never, \*\*ever \*\*heard of a flashing CHECK ENGINE light meaning pull over immediately. A steady OIL warning light sure. In fact, the only time a CHECK ENGINE light should blink is to tell you its code w/o having to use a meter (it will blink a two-digit number like Morse code) and only if you turn the ignition key on & off a certain number of times to make it do that (and this only works on older cars).
Definitely try getting a new (aftermarket) gas cap first…
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### Author:  [@control-z](https://boards.straightdope.com/u/control-z)
#### Post date: [January 15, 2015, 8:15pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/17 "2015-01-15T20:15:35Z")
> [@gigi](#):
>
> Thanks, **GaryT**! I got it through amazon but it says Genuine Toyota so hopefully that’s legit.
>
> [Amazon.com: Toyota Fuel Cap - 77300-06040 : Automotive](http://www.amazon.com/gp/product/B005BU5DCO/)
Yeah that’s the one I found, Amazon says it is the right one and they will let you return it if it doesn’t.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [January 15, 2015, 8:23pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/18 "2015-01-15T20:23:53Z")
> [@Hail\_Ants](#):
>
> I’m a car guy and I have never, \*\*ever \*\*heard of a flashing CHECK ENGINE light meaning pull over immediately. A steady OIL warning light sure. In fact, the only time a CHECK ENGINE light should blink is to tell you its code w/o having to use a meter (it will blink a two-digit number like Morse code) and only if you turn the ignition key on & off a certain number of times to make it do that (and this only works on older cars).
Google “flashing MIL”.
[Edmunds:](http://www.edmunds.com/car-care/what-your-check-engine-light-is-telling-you.html)
> [@Edmunds](#):
>
> Check Engine lights come in orange, yellow or amber, depending on the manufacturer. If the light begins flashing, however, it indicates a more serious problem, such as a misfire that can quickly overheat the catalytic converter.
[Wikipedia:](http://en.wikipedia.org/wiki/Malfunction_indicator_lamp)
> [@Wikipedia](#):
>
> On vehicles equipped with OBD-II, the light has two stages: steady (indicating a minor fault such as a loose gas cap or failing oxygen sensor) and flashing (indicating a severe fault, that could potentially damage the catalytic converter if left uncorrected for an extended period).
You may also wish to read your car’s owner manual, which probably tells you pretty much the same thing. Example, from a [2014 Nissan Maxima, page 2-16: (PDF file)](https://owners.nissanusa.com/content/techpub/ManualsAndGuides/Maxima/2014/2014-Maxima-owner-manual.pdf)
> [@Nissan](#):
>
> Malfunction indicator light blinking—An engine misfire has been detected which may damage the emission control system. To reduce or avoid emission control system damage:
>
> – do not drive at speeds above 45 MPH (72 km/h).
> – avoid hard acceleration or deceleration.
> – avoid steep uphill grades.
> – if possible, reduce the amount of cargo being hauled or towed.
>
> The malfunction indicator light may stop blinkingand come on steady. Have the vehicle inspected by a NISSAN dealer. You do not need to have your vehicle towed to the dealer.
>
> CAUTION
> Continued vehicle operation without having
> the emission control system checked
> and repaired as necessary could lead to
> poor driveability, reduced fuel economy,
> and possible damage to the emission control
> system.
OK, so I guess you can continue to drive at slow speeds, but they seem to admit that damage may be possible even at speeds under 45 MPH, noting that this may only “reduce” damage.
TL,DR: a steady MIL can be ignored indefinitely without imperiling the vehicle. Not so for a flashing MIL.
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### Author:  [@jz78817](https://boards.straightdope.com/u/jz78817)
#### Post date: [January 15, 2015, 9:21pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/19 "2015-01-15T21:21:04Z")
FWIW a flashing CEL usually means “repeated misfire.” the added raw fuel/air charge that isn’t getting burned in the cylinder gets burned in the catalyst instead, which can destroy the substrate in short order.
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### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [January 15, 2015, 10:22pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/20 "2015-01-15T22:22:59Z")
> [@Hail\_Ants](#):
>
> IANAL but I don’t see how the State could **possibly** stop Auto Zone from doing this, they are not violating any law, and any law passed saying that they are would be unconstitutional IMO.
Apparently you’re not familiar with California, its stringent emissions regulations, and its BAR (Bureau of Auto Repair). Any business performing procedures on motor vehicles – and yes, reading trouble codes is a procedure – is required to have an auto repair shop license. This entails jumping through a bunch of hoops, meeting a bunch of conditions, and paying money. Auto parts stores in that state have decided it’s not worth it to them to become so licensed, so they don’t read codes. It’s no more unconstitutional than requiring other businesses to be licensed.
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### Author:  [@Hampshire](https://boards.straightdope.com/u/Hampshire)
#### Post date: [January 15, 2015, 10:38pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/21 "2015-01-15T22:38:28Z")
I had an older Corolla that was doing this. While the error code indicated that it was the the old “gas cap” problem in reality it was a faulty sensor saying there was something bad with the gas cap.
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