# 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:** 15\ **Page:** 3
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [January 17, 2015, 1:46am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/42 "2015-01-17T01:46:01Z")
> [@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! It seems like getting behind the dash to unplug the light would be more work than actually fixing most problems that trigger a CEL. ---
**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, 2:00am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/43 "2015-01-17T02:00:25Z")
> [@GreasyJack](#): > > It seems like getting behind the dash to unplug the light would be more work than actually fixing most problems that trigger a CEL. not if the cause of a CEL is a circuit fault instead of a failed part. tracing circuit faults in a typical wiring harness bundle can be a right pain in the ass. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 17, 2015, 4:33am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/44 "2015-01-17T04:33:17Z")
> [@razncain](#): > > 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. This is a prime example of why when somebody says just read the code and replace the part those of us that do this stuff for living shake our heads. It ain’t that easy folks. Analyzing O2 sensor signals front and rear as well as running parameters to determine if it is a cat and which one is a skill. Your car has 3 cats so you have either 3 or 4 O2 sensors to read as well as the other running parameters. What kind of car is it and what codes have set? ---
**Author:** ![Gatopescado](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gatopescado/32/5711_2.png) [@Gatopescado](https://boards.straightdope.com/u/Gatopescado)\ **Post date:** [January 17, 2015, 4:48am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/45 "2015-01-17T04:48:51Z")
> [@control-z](#): > > And yet the car kept running fine? My 97 BMW has had the “Check Engine” light on since 2001. Runs better every day, I swear. ---
**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 17, 2015, 4:54am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/46 "2015-01-17T04:54:19Z")
> [@Gatopescado](#): > > My 97 BMW has had the “Check Engine” light on since 2001. Runs better every day, I swear. Just don’t have Sheldon as a passenger… ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 17, 2015, 5:42am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/47 "2015-01-17T05:42:18Z")
> [@jz78817](#): > > not if the cause of a CEL is a circuit fault instead of a failed part. tracing circuit faults in a typical wiring harness bundle can be a right pain in the ass. Harness faults are fairly easy to diagnosis. You check the wire in question for continuity, short to ground, short to power finds about 95%+ of all harness faults. Harness faults come in three general flavors. 1. At a connector. Bent pin, backed out pin, corrosion. Easy to fix. 2. Failed harness due being physically damaged. If it was routed wrong at the factory and a sharp bracket or something similar cut the harness. Finding it can be hard. Repair can be easy, but in severe cases the harness must be replaced. 3. Rodents. Usually easy to fix. They eat a couple of wires and quit. Again repair or replace. ---
**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, 1:27pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/48 "2015-01-17T13:27:13Z")
> [@Rick](#): > > Harness faults are fairly easy to diagnosis. You check the wire in question for continuity, short to ground, short to power finds about 95%+ of all harness faults. > > Harness faults come in three general flavors. > > 1. At a connector. Bent pin, backed out pin, corrosion. Easy to fix. > 2. Failed harness due being physically damaged. If it was routed wrong at the factory and a sharp bracket or something similar cut the harness. Finding it can be hard. Repair can be easy, but in severe cases the harness must be replaced. > 3. Rodents. Usually easy to fix. They eat a couple of wires and quit. Again repair or replace. The car I referred to above was notorious for “rub-throughs” (wires abraded their insulation away and shorted together) and invisible breaks (copper wire breaks but the insulation stays intact) inside the main engine harness. Picking apart that bundle and finding the suspect wires was not fun, especially with so little room to work. And there must be something unique about the high temp insulation used on O2 sensors. Rodents _love_ to chew on those things. ---
**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 18, 2015, 5:36am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/49 "2015-01-18T05:36:00Z")
> [@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! It must have been an older model, as modern cars will not run if the CHECK ENGINE bulb in the dashboard is removed. of course you can cover the lens in the dash with black electrical tape etc. ---
**Author:** ![usedtobe](https://avatars.discourse-cdn.com/v4/letter/u/ec9cab/32.png) [@usedtobe](https://boards.straightdope.com/u/usedtobe)\ **Post date:** [January 18, 2015, 5:40am UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/50 "2015-01-18T05:40:43Z")
> [@Hail\_Ants](#): > > It must have been an older model, as modern cars will not run if the CHECK ENGINE bulb in the dashboard is removed. … Does anyone else find this scary? “I can’t do that, Dave”… Every day brings news of another hack - meanwhile, we rush to “the internet of things” - I can’t wait for the first hack of a cloud-controlled train (why bother stopping? or activating the grade-crossing wig-wags?) ---
**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 18, 2015, 12:07pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/51 "2015-01-18T12:07:05Z")
> [@Hail\_Ants](#): > > It must have been an older model, as modern cars will not run if the CHECK ENGINE bulb in the dashboard is removed. Not true at all – that is just flat wrong. I don’t know where you get these ideas, but the misinformation is not helping. ---
**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 18, 2015, 1:15pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/52 "2015-01-18T13:15:51Z")
> [@Gary\_T](#): > > Not true at all – that is just flat wrong. I don’t know where you get these ideas, but the misinformation is not helping. the only way unplugging the check engine light will cause trouble is if you live in an area which mandates emissions inspections. the first thing they’re supposed to do is verify that the check engine light is lit when the cluster does the brief “tell-tale” mode\* after you turn the car on. If it isn’t, your car fails. - “tell-tale” mode is when the gauge cluster turns on all of the warning lights for a few seconds after turning the key to RUN so you can verify none of the bulbs or LEDs are burned out. some cars also have a test mode for the cluster where you can call up the tell-tale on demand. ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [January 19, 2015, 5:24pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/53 "2015-01-19T17:24:53Z")
The light had turned off again when I started up on Saturday – I will still put in the new gas cap and wait and see. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [January 19, 2015, 5:56pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/54 "2015-01-19T17:56:05Z")
> [@gigi](#): > > The light had turned off again when I started up on Saturday – I will still put in the new gas cap and wait and see. Your car and my car must be talking to one another. My 2009 Elantra has been doing the same thing for maybe 4 to 6 weeks now. What I think I noticed though, is that it seems to toggle on when the outside temperature changes significantly. I may be imagining it, or it may be coincidental, but it seems like the light will go off after a few days of driving and stay off until the next major temperature delta whereby it turns on again for three or four days. Lather, rinse, repeat. I imagine the weather in NH has been swinging a lot this winter, as in Ottawa. ---
**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 19, 2015, 7:55pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/55 "2015-01-19T19:55:10Z")
> [@Hail\_Ants](#): > > 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. Engines don’t shut down when there’s a indication of low oil pressure, which is generally understood (even by non car guys) to be an engine-destroying condition. So why would the engine deliberately shut down for a flashing MIL? Yes, a flashing MIL is rare, because that sort of problem (severe, persistent misfire that sends large quantities of unburned mixture to the cat, overheating it) is rare. It’s not necessary to see or hear of it happening in order to understand what a flashing MIL means; all you need to do is read the owner’s manual, which any person in the internet who boasts “I’m a car guy” ought to know. FWIW, if someone’s car deliberately shut itself down in the middle of Death Valley because they got a flashing MIL, there would be a massive lawsuit over the resulting deaths. > [@Hail Ants](#): > > It must have been an older model, as modern cars will not run if the CHECK ENGINE bulb in the dashboard is removed. of course you can cover the lens in the dash with black electrical tape etc. You really should stop participating in this thread; the Dope’s reputation for straightness is suffering because you keep spreading misinformation. Modern cars will run just fine, no matter what you do to the MIL bulb, but they’ll [throw a code,](http://www.obd-codes.com/p0650) and of course the MIL won’t light up at key-on, which is pretty much a dead giveaway. ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [January 21, 2015, 2:47pm UTC](https://boards.straightdope.com/t/stupid-check-engine-light/709870/56 "2015-01-21T14:47:05Z")
> [@Leaffan](#): > > Your car and my car must be talking to one another. My 2009 Elantra has been doing the same thing for maybe 4 to 6 weeks now. Despite the many miles separating them, my Corolla has always felt that it could tell when your Elantra was sick, injured or just upset. I guess this seals it. [Previous page](https://boards.straightdope.com/t/stupid-check-engine-light/709870.md?page=2)