# Stupid Check Engine Light **URL:** \ **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:** 2
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [January 15, 2015, 11:02pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/22 "2015-01-15T23:02:56Z")
> [@Hail\_Ants](#): > > I’m a car guy and I have never, \*\*ever \*\*heard of a flashing CHECK ENGINE light meaning pull over immediately. Huh. The shops told me this, and a co-worker with the same issues was told the same thing. > [@Machine\_Elf](#): > > 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. That makes me feel a little better – I was picturing having to pull over on the highway and DRIVE NO FURTHER. Roads are not heavily populated here so I would be able to drive slowly to the repair shop near me without too much risk to others. **Hampshire** , thanks for the reminder about the sensor too. Someone else mentioned that in passing but it could the next step if the cap doesn’t help. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 16, 2015, 1:42am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/23 "2015-01-16T01:42:00Z")
> [@SmellMyWort](#): > > 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. Or less than $10 if you have a smartphone: [http://www.amazon.com/Version-Bluetooth-Diagnostic-Interface-Scanner/dp/B008UR7J6I/ref=sr\_1\_5?s=automotive&ie=UTF8&qid=1421372215&sr=1-5&keywords=elm327&tag=viglink20265-20](http://www.amazon.com/Version-Bluetooth-Diagnostic-Interface-Scanner/dp/B008UR7J6I/ref=sr_1_5?s=automotive&ie=UTF8&qid=1421372215&sr=1-5&keywords=elm327&tag=viglink20265-20) In addition to the codes, those can send realtime sensor data to your phone. Nifty! ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 16, 2015, 1:48am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/24 "2015-01-16T01:48:13Z")
> [@Joey\_P](#): > > 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 depends on the car. On some, they use passive leak detection where it looks for a change in pressure in the tank as the outside temperature changes. So, basically, when the car is sitting overnight. On other cars, though, they have an active system that has a pump that slightly pressurizes the tank to test the integrity of the tank after the car has been parked for a few hours. I _think_ this vintage of Toyota has an active system, but I’m not 100% sure. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 16, 2015, 2:02am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/25 "2015-01-16T02:02:20Z")
FWIW, I had a Mustang that failed emissions because of an Evaporative Emissions Control failure. I replaced the gas cap, and it still failed. So, I got under the car, and found that at some point I had drove over something tall enough to snag the metal emissions vent tube and bend it, at which point it popped out of the rubber tube it mated with. I bent it back as good as I could, cut a longer piece of rubber tubing to replace the original one, and it passed the next time. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 16, 2015, 2:12am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/26 "2015-01-16T02:12:56Z")
> [@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. > > So yes, just take it to Auto Zone to have the code read. > > 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… Umm yes they were violating the law. In California to diagnosis and repair automobiles you must be a licensed Automotive Repair Dealer (ARD). California has the strongest consumer protection laws in the country when it comes to auto repair. They protect the consumer and the shop. So what would happen is customer would take car to Auto Zone they would read code. The next thing out of the customer’s mouth would be what do I need to buy to fix this? As soon as the employee answers he is interpreting and diagnosing the car. If he is right it is kind of a no harm no foul deal. The problem is they didn’t have a very good batting average. What happened was there was lots of piss poor diagnostics done and some customers laid out $1,000s of dollars on unneeded parts. Complaints to the Bureau of Automotive Repair soon followed. The state takes a very dim view of that shit so they went after Auto Zone for being an unlicensed ARD. Damn there goes the state trying to protect the consumer again. Who do they think they are? I can give you numerous examples of read the code and misdiagnosis the car right here on this board. Oh and has been mentioned up thread don’t buy an aftermarket cap as they don’t always work. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 16, 2015, 2:57am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/27 "2015-01-16T02:57:59Z")
nevermind describing oneself as a “car guy” and not knowing a flashing CEL means “stop the car as soon as possible.” ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [January 16, 2015, 3:12am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/28 "2015-01-16T03:12:57Z")
> [@GreasyJack](#): > > It depends on the car. On some, they use passive leak detection where it looks for a change in pressure in the tank as the outside temperature changes. So, basically, when the car is sitting overnight. On other cars, though, they have an active system that has a pump that slightly pressurizes the tank to test the integrity of the tank after the car has been parked for a few hours. I _think_ this vintage of Toyota has an active system, but I’m not 100% sure. I didn’t know if it had anything to do with the temperatures being at their lowest these days. I get 87 gas and my co-worker says get the “middle” one (89?) and that will help with the CEL. I can’t imagine it would matter?? ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 16, 2015, 3:23am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/29 "2015-01-16T03:23:42Z")
> [@GreasyJack](#): > > It depends on the car. On some, they use passive leak detection where it looks for a change in pressure in the tank as the outside temperature changes. So, basically, when the car is sitting overnight. On other cars, though, they have an active system that has a pump that slightly pressurizes the tank to test the integrity of the tank after the car has been parked for a few hours. I _think_ this vintage of Toyota has an active system, but I’m not 100% sure. This is correct. Some systems pull a slight vacuum while the car is running. Others pressurize the tank with the engine running, still others pressurize the tank after engine shut down. It depends on the car maker model year and model. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 16, 2015, 3:34am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/30 "2015-01-16T03:34:25Z")
> [@gigi](#): > > I didn’t know if it had anything to do with the temperatures being at their lowest these days. > > I get 87 gas and my co-worker says get the “middle” one (89?) and that will help with the CEL. I can’t imagine it would matter?? It doesn’t really matter what the temperature is, just that there’s enough of a variation between daytime and nighttime temperature to create a pressure difference in the tank. (If you live in some place with a screwy climate that has long periods where there isn’t much day to night variation, it won’t cause the light to go on but the computer won’t register a successful evaporative emissions test and so you’d fail a plug-in emissions test.) Like I said, though, I think your Toyota has an active system that just periodically pressurizes the tank with a pump so that isn’t relevant at all. And, no, the fuel won’t help. If you had a car that required premium gas, running regular _might_ cause a CEL on some cars, but I’m pretty sure your Corolla only needs regular. ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [January 16, 2015, 3:54am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/31 "2015-01-16T03:54:33Z")
Thanks! ---
**Author:** ![carnut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnut/32/3531_2.png) [@carnut](https://boards.straightdope.com/u/carnut)\ **Post date:** [January 16, 2015, 4:02am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/32 "2015-01-16T04:02:00Z")
> [@jz78817](#): > > Stop giving bad advice. Maybe stop reading “advice” into what I wrote. I didn’t say “change it,” I said “perhaps it is” and described what it was for my situation. But thank you for your direct order. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 16, 2015, 4:24am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/33 "2015-01-16T04:24:36Z")
> [@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. Either this is the stupidest automotive post I have ever read on this message board or… Nope this the stupidest automotive post ever. I promise you there is NO way in hell an evap code could be triggered by a bad converter. Either there were other codes you aren’t telling us about or a second problem reared its head after the dealer read the evap code. So since replacing a converter is nowhere in the fault tree for any evap code what pray tell led you to a converter replacement? ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 16, 2015, 12:45pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/34 "2015-01-16T12:45:19Z")
> [@gigi](#): > > I get 87 gas and my co-worker says get the “middle” one (89?) and that will help with the CEL. I can’t imagine it would matter?? You’re right. For any code that could be caused by a faulty cap, the octane rating is irrelevant. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 16, 2015, 6:49pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/35 "2015-01-16T18:49:10Z")
> [@gigi](#): > > That makes me feel a little better – I was picturing having to pull over on the highway and DRIVE NO FURTHER. Roads are not heavily populated here so I would be able to drive slowly to the repair shop near me without too much risk to others. Don’t take my word for it, or Nissan’s. Read the owner’s manual for _your_ car and find out how Toyota recommends you deal with a flashing MI; For your 2008 Corolla, they may indeed advise stopping immediately, or setting a speed limit far less than 45MPH, or something else. ---
**Author:** ![Razncain](https://avatars.discourse-cdn.com/v4/letter/r/9d8465/32.png) [@Razncain](https://boards.straightdope.com/u/Razncain)\ **Post date:** [January 16, 2015, 8:36pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/36 "2015-01-16T20:36:38Z")
Often codes can only take you so far. Started having this problem when I switched out the standard 2004 Ford Ranger gas cap with one that had the lock and key. It doesn’t come on all of the time, and generally it resets. Comes on now for something unrelated to gas cap. Have had three different mechanics tell me emissions, with all stating it was a catalytic convertor problem. Mechanic had me take it to an exhaust shop, they said the only way to know for sure was to burn and blow holes in exhaust pipe and do tests. He said the bad news was, I have three catalytic convertors. The good news is, only one is bad. Replaces the one he said is bad. After another 70 miles check engine comes back on again. Exhaust shop didn’t know what to say. Took it back to original mechanic, and he ran codes again, still indicating a particular one was bad. Not sure which one they replaced. He wants me to come back in towards the end of the month to have it tested on a new code reader that is supposed to be better than the other ones. We’ll see. ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [January 16, 2015, 8:45pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/37 "2015-01-16T20:45:31Z")
> [@Machine\_Elf](#): > > Don’t take my word for it, or Nissan’s. Read the owner’s manual for _your_ car and find out how Toyota recommends you deal with a flashing MI; For your 2008 Corolla, they may indeed advise stopping immediately, or setting a speed limit far less than 45MPH, or something else. Yes, point taken. ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [January 16, 2015, 11:38pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/38 "2015-01-16T23:38:13Z")
Apparently I’m incorrect regarding the flashing check engine light. It must be fairly rare as like I said I’ve never seen nor heard of one doing this. I assumed a modern car’s ECU would simply engage a ‘kill switch’ for any serious, engine-damaging malfunctions. And some high-end luxury cars could engage a ‘limp-home’ mode. > [@Rick](#): > > Umm yes they were violating the law. In California to diagnosis and repair automobiles you must be a licensed Automotive Repair Dealer (ARD). California has the strongest consumer protection laws in the country when it comes to auto repair. They protect the consumer and the shop. > So what would happen is customer would take car to Auto Zone they would read code. > The next thing out of the customer’s mouth would be what do I need to buy to fix this? As soon as the employee answers he is interpreting and diagnosing the car. > If he is right it is kind of a no harm no foul deal. The problem is they didn’t have a very good batting average. What happened was there was lots of piss poor diagnostics done and some customers laid out $1,000s of dollars on unneeded parts. Complaints to the Bureau of Automotive Repair soon followed. > The state takes a very dim view of that shit so they went after Auto Zone for being an unlicensed ARD. Damn there goes the state trying to protect the consumer again. Who do they think they are? > I can give you numerous examples of read the code and misdiagnosis the car right here on this board. > > Oh and has been mentioned up thread don’t buy an aftermarket cap as they don’t always work. Yeah know I was going to sarcastically say, “I figured it had to be California…” but I’ll admit your post does make it sound somewhat reasonable. Still, as a ‘rugged individualist’ I would still firmly remain on the _caveat emptor_ side of things. ---
**Author:** ![DingoelGringo](https://avatars.discourse-cdn.com/v4/letter/d/9d8465/32.png) [@DingoelGringo](https://boards.straightdope.com/u/DingoelGringo)\ **Post date:** [January 16, 2015, 11:42pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/39 "2015-01-16T23:42:40Z")
> [@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. 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.) I took a car into the dealer with an erratic “Check Engine” light and the mechanic fixed it quickly. 3 years later I discovered he’d fixed it by disconnecting the bulb! ---
**Author:** ![control-z](https://avatars.discourse-cdn.com/v4/letter/c/eada6e/32.png) [@control-z](https://boards.straightdope.com/u/control-z)\ **Post date:** [January 17, 2015, 12:07am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/40 "2015-01-17T00:07:05Z")
> [@DingoelGringo](#): > > I took a car into the dealer with an erratic “Check Engine” light and the mechanic fixed it quickly. 3 years later I discovered he’d fixed it by disconnecting the bulb! And yet the car kept running fine? There are so many different things that can trip a check engine light that I don’t worry too much when I see one. I pay attention to how my vehicle sounds and drives, and the gas mileage. If all is about the same I figure it can wait. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 17, 2015, 12:31am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/41 "2015-01-17T00:31:08Z")
> [@control-z](#): > > And yet the car kept running fine? > > There are so many different things that can trip a check engine light that I don’t worry too much when I see one. I pay attention to how my vehicle sounds and drives, and the gas mileage. If all is about the same I figure it can wait. many common things that trigger a CEL don’t cause driveability problems. But since OBD-II is a legislated requirement, the CEL has to be lit any time the PCM detects a fault that can cause an increase in the vehicle’s emissions. Some faults that will light the CEL but not cause any driveability problems are things like evap faults (loose gas cap is one possible cause) or catalyst efficiency (the catalytic converter isn’t working anymore.) On past cars I’ve personally had a couple of faults which put on the CEL _and_ caused driveability problems; one was a P0509 Idle Air Control Circuit High (car wouldn’t idle,) and a P0340 Camshaft Position Sensor Circuit Malfunction (car took a lot longer to start and would stumble and misfire above 2500 rpm.) [Previous page](https://boards.straightdope.com/t/stupid-check-engine-light/709870.md?page=1) [Next page](https://boards.straightdope.com/t/stupid-check-engine-light/709870.md?page=3)