# What does (or could) a "check engine light" being on, mean in modern day cars? **URL:** \ **Category:** Factual Questions\ **Created:** [October 9, 2016, 6:20am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037 "2016-10-09T06:20:40Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Flyer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/flyer/32/7298_2.png) [@Flyer](https://boards.straightdope.com/u/Flyer)\ **Post date:** [October 9, 2016, 2:53pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/21 "2016-10-09T14:53:48Z")
As an aside, this is one of the most interesting changes between old cars and new cars. In old cars, the engine/temp light meant “pull over RIGHT NOW, or risk ruining the engine.” In new cars, the engine light means “something is probably mildly wrong with any of 5 dozen different systems, only a handful of which actually have anything to do with the actual engine.” (Possibly slightly exaggerated. :)) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 9, 2016, 3:01pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/22 "2016-10-09T15:01:55Z")
Not quite. In old cars there were 1 or 2 lights. Either of which came on for immediately expensive bad news as you say. In new cars there are 3 lights. The same two as always that mean the same dire problems as they always did. Plus a third light that means “No biggee, but Big Brother will have harsh words for you at registration renewal time.” The other difference is that in a modern car, the first two lights will never come on; statistically speaking those defects just don’t happen. That third light is probably the only idiot light most drivers under age 50 have ever seen come on while driving. Hence their confusion. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [October 9, 2016, 3:20pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/23 "2016-10-09T15:20:03Z")
> [@Crazyhorse](#): > > Yeah, while it technically \*could \*mean anything, the context makes it unlikely that it’s something like a loose gas cap or a catastrophic engine problem. > > A check engine light mentioned in an ad for a car that is otherwise presented as being all tuned up and running fine probably means it can’t pass a smog check and may need a new catalytic converter or something to pass inspection. More often than not it’s the O2 sensor. Should be easy enough to check, and if it’s the O2 sensor easy enough to fix (by a professional). ---
**Author:** ![albino\_manatee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/albino_manatee/32/2_2.png) [@albino\_manatee](https://boards.straightdope.com/u/albino_manatee)\ **Post date:** [October 9, 2016, 3:25pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/24 "2016-10-09T15:25:03Z")
i had a 2002-honda-civic … the “cel” appeared directly after factory-warranty ran out. i did nothing about the cel … and continued driving her into the ground. total of 425,000 miles on the odometer, before some shady mechanic ‘forgot’ to replace the thermostat when replacing timing-belts. i am now driving a 2015-honda-civic with 38,000 miles on the odometer … fully expecting the cel to display once the warranty expires. p.s. and, yes, she passed yearly vehicle inspections. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [October 9, 2016, 3:27pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/25 "2016-10-09T15:27:48Z")
My check-engine light lit recently on my MB for no other reason than to force me in to the dealer so that they could download new software for the car. :dubious: ---
**Author:** ![cochrane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cochrane/32/10441_2.png) [@cochrane](https://boards.straightdope.com/u/cochrane)\ **Post date:** [October 9, 2016, 7:30pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/26 "2016-10-09T19:30:14Z")
> [@LSLGuy](#): > > All the above commentary about various technical details is correct as far as it goes. But it misses the larger point. > > \*The \* key decision-making issue for the OP is what city & state is he in? Does that location have emissions checks? What sort of checks? > > If not, the CEL and underlying problem is \*probably \*ignorable. For a while. It may also indicate a car whose fault causes crappier than normal gas mileage. Which may offset the lower price you can get for buying a CEL-on car. > > If the OP is in a location with stringent emissions checks the situation changes completely. > > If so, now he has bought an unregisterable car with an unknown cost-to-repair before it can be registered. A code reader plus Google, or better yet a local mechanic, can \*estimate \*how hard/expensive fixing this particular code or codes \*probably \*will be. But with pretty large error bars. And the bigger the potential repair, the bigger the corresponding uncertainty. > > That’s about of like agreeing to buy a used car for some number between $1000 and $4000 based on rolling some dice after you’ve shook hands on the deal. IOW, some people might take that gamble; experienced car people won’t. Getting a mechanic’s inspection reduces the amount at risk, but doesn’t eliminate it. The OP said he is moving to Hawaii. Hawaii doesn’t require emissions testing. > **[Vehicle Emissions Testing in Hawaii | DMV.com](https://www.dmv.com/hi/hawaii/emissions-testing)** > > HI emission tests and standards, detailing contact information and requirements In Hawaii, a vehicle emissions test is not required for original registrations, as well as for registration renewals. The Honolulu Clean Cities program focuses on... ---
**Author:** ![usedtobe](https://avatars.discourse-cdn.com/v4/letter/u/ec9cab/32.png) [@usedtobe](https://boards.straightdope.com/u/usedtobe)\ **Post date:** [October 9, 2016, 9:30pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/27 "2016-10-09T21:30:42Z")
In CA, Auto Zone (auto parts chain) would connect the plug and tell you what the problem was. Since auto repair shops were getting $80.00 to do the same thing, the mechanics complained, the State sent a nasty letter, Auto Zone stopped doing the diagnostic. Upshot: I now have a ($120) tool which reads the OBDII system and displays error code(s). It will also reset the “Check Engine” light. IOW: the seller almost certainly knows why the light is on. And prefers to sell without fixing it. I’d take my little “magic box” and read the codes myself. If having an OBDII reader (is OBDII still latest and greatest, or is there now an OBDIII?) does not appeal to you, have the car checked. But only if you really, really want THAT car. My opinion of someone who would sell a car in that condition is such as I would not even consider the car until a mechanic (not me, a mechanic) says the OBDII system is good. The difference? Remember the part about the little box being able to reset the light? There are other ways which will also cancel the light. I won’t trust me to know if the thing is in good shape or just jerry-rigged to cancel the light. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 9, 2016, 10:03pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/28 "2016-10-09T22:03:37Z")
> [@cochrane](#): > > The OP said he is moving to Hawaii. Hawaii doesn’t require emissions testing. > > [Vehicle Emissions Testing in Hawaii | DMV.com](https://www.dmv.com/hi/hawaii/emissions-testing) Good news. He’s in like Trump then. Or something. ---
**Author:** ![zwede](https://avatars.discourse-cdn.com/v4/letter/z/ac91a4/32.png) [@zwede](https://boards.straightdope.com/u/zwede)\ **Post date:** [October 9, 2016, 10:09pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/29 "2016-10-09T22:09:21Z")
> [@bardos](#): > > I’ve been checking out code readers and watching tutorials on youtube. Very interesting all of this. Is this a recommended check on checking out any car, even with no check light on? Yes, absolutely. Say the crank position sensor (a very essential sensor to make the engine run) was sending wonky signals and set a code, but it was only doing it when it was really hot. Now the weather is cooler and the signal is OK. The MIL (“check engine”) light will be off since there have been multiple drive cycles with good signal. A good scan tool will let you look at the fault history stored by the computer. Even though there’s no active code you’ll see it in the history and you’ll know you will most likely have to replace that sensor before too long. ETA: Correct term is MIL, Malfunction Indicator Light. Even though many auto manufacturers write “check engine” on the light itself. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [October 9, 2016, 11:19pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/30 "2016-10-09T23:19:38Z")
It might be worth noting that while an OBD II reader is a very useful tool, it has somewhat limited utility for the average person who doesn’t know a lot about cars, like me, a certified doofus when it comes to cars. The problem is that there are [hundreds of DTC codes](https://www.obd-codes.com/trouble_codes/) and most of them are pretty technical and obscure; furthermore, a single problem may trigger multiple codes. IOW: it’s not a deep intelligence that clearly tells you what’s wrong – it’s an analytical tool that requires a good deal of knowledge. The only way to know definitively what’s wrong is to have the car checked out by a competent mechanic, who will read the trouble codes as a _starting point_ in his diagnosis. Still, I have one and it’s one of those things that I rarely use but am really happy that I have it on those rare occasions when I do need it. I mostly use it to confirm that a car is ready for the emissions test that is required every two years here, especially if it’s had a repair or the battery has been disconnected and I need to verify that the non-continuous monitors have come ready, which can take several days of different patterns of driving. And, of course, in many cases, with the aid of The Google, you actually can confirm that the “Check Engine” idiot light is due to something relatively trivial, like **John** ’s example of one of the common O2 sensor codes – you just can’t count on it. ---
**Author:** ![John\_Bredin](https://avatars.discourse-cdn.com/v4/letter/j/96bed5/32.png) [@John\_Bredin](https://boards.straightdope.com/u/John_Bredin)\ **Post date:** [October 9, 2016, 11:28pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/31 "2016-10-09T23:28:06Z")
Asking a related question without starting a new thread, what could cause the “check coolant” light to come on but **not** produce a code nor move the engine-temp gauge beyond the (presumably safe) middle of the dial? ---
**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\ **Post date:** [October 9, 2016, 11:32pm UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/32 "2016-10-09T23:32:49Z")
My CEL came on about a month ago. Stopped by my mechanic to have it checked. It was an error in a transfer case solenoid, it had nothing to do with the emissions system. My mechanic reseated the connector, cleared to code and all has been good since. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 10, 2016, 12:02am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/33 "2016-10-10T00:02:27Z")
> [@John\_Bredin](#): > > Asking a related question without starting a new thread, what could cause the “check coolant” light to come on but **not** produce a code nor move the engine-temp gauge beyond the (presumably safe) middle of the dial? I would not expect a coolant level issue to produce a code. Nor to set the MIL on. My vehicle has a coolant level sensor. That lights the light when the coolant is a bit low. Well before enough is lost to cause any issues with temperature control. So step one is to check coolant levels and refill as necessary. Follow the owner’s manual procedures since hot coolant can be dangerous if you’re clueless about how to operate your car’s radiator system. I’m amazed they still make some cars that can’t be refilled hot, but there are some. If it needs fluid the light is working correctly. If it doesn’t then you have a mystery about why the sensor is triggering for no good reason. Until the false alarms get so frequent as to be an irritant the problem can be ignored. OTOH, if it keeps needing refilling then we know that the coolant is going someplace. Which might just be onto the ground which is cheap. Or into the combustion chambers or crank case sump which is expensive. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [October 10, 2016, 12:11am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/34 "2016-10-10T00:11:33Z")
> [@John\_Bredin](#): > > Asking a related question without starting a new thread, what could cause the “check coolant” light to come on but **not** produce a code nor move the engine-temp gauge beyond the (presumably safe) middle of the dial? Some cars periodically do things like this for routine maintenance items. Basically, every so many miles (whatever they’ve programmed it for) it will light that indicator to tell you that you are due to have your coolant flushed and refilled. What type of car is it? You can probably find out from google if the car has a low coolant sensor (like **LSLGuy** referred to) or if it is one of those lights that is pre-programmed to come on even if you don’t have a problem. ---
**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:** [October 10, 2016, 1:34am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/35 "2016-10-10T01:34:22Z")
> [@usedtobe](#): > > In CA, Auto Zone (auto parts chain) would connect the plug and tell you what the problem was. no, they would read the code and tell you what the code says. the code doesn’t necessarily tell you what the problem is. > [@racer72](#): > > My CEL came on about a month ago. Stopped by my mechanic to have it checked. It was an error in a transfer case solenoid, it had nothing to do with the emissions system. My mechanic reseated the connector, cleared to code and all has been good since. two things: 1. the OBD-II standards define the minimum situations which a car _must_ activate the check engine light. they’re free to use the CEL for other faults. 2. transmission/transfer case problems can indeed have emissions implications. unexpectedly abrupt gear shifts or engagements can cause the engine to burp out excessive pollutants. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 10, 2016, 3:00am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/36 "2016-10-10T03:00:34Z")
Yup. I have a Jeep Liberty that developed transmission weirdness last year. And the MIL at about the same time. Turned out to be a fairly common problem on that model. Some gizmo gets sticky in the trans, interferes with the shifting logic, and the MIL lights because running around at redline in second instead of shifting into third is something you really ought to get fixed. $50 for parts, $100 for labor and she’s good as new. ---
**Author:** ![Emtar\_KronJonDerSohn](https://avatars.discourse-cdn.com/v4/letter/e/58956e/32.png) [@Emtar\_KronJonDerSohn](https://boards.straightdope.com/u/Emtar_KronJonDerSohn)\ **Post date:** [October 10, 2016, 4:12am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/37 "2016-10-10T04:12:09Z")
> [@engineer\_comp\_geek](#): > > I always assume that if the check engine light is on, the owner already tried to have it fixed (since most folks don’t want a car that won’t pass emissions) and failed to fix it for a reasonable price. +1 It’s in the same category as “my cousin gave me the car without a title but you can order one at the DMV for like $10 or something, it’s real easy but I haven’t got around to it yet.” If it was really nothing the seller would have fixed it already to sell at full price without an asterisk. ---
**Author:** ![Crazyhorse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crazyhorse/32/31_2.png) [@Crazyhorse](https://boards.straightdope.com/u/Crazyhorse)\ **Post date:** [October 10, 2016, 4:56am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/38 "2016-10-10T04:56:54Z")
> [@Emtar\_KronJonDerSohn](#): > > +1 > > It’s in the same category as “my cousin gave me the car without a title but you can order one at the DMV for like $10 or something, it’s real easy but I haven’t got around to it yet.” > > If it was really nothing the seller would have fixed it already to sell at full price without an asterisk. Not if they are selling it in a state with no emissions testing and the only reason for the CEL is a bad o2 sensor or catalytic converter. It’s just a minor annoyance to anyone not subject to emissions standards. They are leaving it up to the new owner to decide how they want to handle it. As long as they have a mechanic check it out and confirm that is the case it’s not a deal breaker. If they wanted to pull a scam they would reset the light and hope it doesn’t come back on until after the test drive. ---
**Author:** ![bardos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bardos/32/1069_2.png) [@bardos](https://boards.straightdope.com/u/bardos)\ **Post date:** [October 10, 2016, 6:44am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/39 "2016-10-10T06:44:52Z")
Back in the old days, I recall one scam on selling a second hand car was to turn back the odometer to show less miles driven. Is the following deduction correct?: now one can reset the light by disconnecting the battery or somehow messing with the codes and the “check engine light” will go out until driven for a few days? If so, it sounds like one can never really know when one appraises a 2nd hand car… Straighten me out here, please. ---
**Author:** ![Crazyhorse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crazyhorse/32/31_2.png) [@Crazyhorse](https://boards.straightdope.com/u/Crazyhorse)\ **Post date:** [October 10, 2016, 6:59am UTC](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037/40 "2016-10-10T06:59:56Z")
> [@bardos](#): > > Back in the old days, I recall one scam on selling a second hand car was to turn back the odometer to show less miles driven. > Is the following deduction correct?: now one can reset the light by disconnecting the battery or somehow messing with the codes and the “check engine light” will go out until driven for a few days? If so, it sounds like one can never really know when one appraises a 2nd hand car… > > Straighten me out here, please. Yes the CEL can be reset with a simple device like those that have been linked in this thread. I don’t know if it will stay off a few days or a few miles or a few hours, but it usually doesn’t come back on immediately. The fact that they are openly telling you the light is on, while (presumably) also representing that the car is in good working order, makes it likely it’s just an emissions error that isn’t applicable in your state. It’s an ‘optional’ repair as far as the mechanical integrity of the car is concerned. (although in some cars there could be a hit on gas mileage to drive it with a bad CC or o2 sensor.) It’s always a good idea to have a trusted mechanic inspect any car you’re seriously considering for many reasons. [Previous page](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037.md?page=1) [Next page](https://boards.straightdope.com/t/what-does-or-could-a-check-engine-light-being-on-mean-in-modern-day-cars/768037.md?page=3)