# My car's" Check Engine" light: Has the sensor gone haywire? **URL:** \ **Category:** Factual Questions\ **Created:** [September 1, 2014, 7:43pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183 "2014-09-01T19:43:02Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![dougie\_monty](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@dougie\_monty](https://boards.straightdope.com/u/dougie_monty)\ **Post date:** [September 1, 2014, 7:43pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/1 "2014-09-01T19:43:02Z")
I drive a '92 Ford Taurus GL with a “Check Engine” light on the instrument panel. It has been lighting up lately, and obligingly I have checked the levels of the oil and coolant. The light, however, still goes on. And here’s the odd part: It will appear when I start out and go south or west, not east or north. So I can drive somewhere to the west or south of me and the light will stay on; but returning northward or eastward, for the same distance, the light doesn’t go on at all. About the time this started happening, my turn signals, which have been working intermittently for months, now work all the time. Well, I’m bringing this to my mechanic’s attention in a few days. Any comments? ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 1, 2014, 7:48pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/2 "2014-09-01T19:48:23Z")
Take it to a auto store like autozone or advanced auto and they will hook up a bod scanner to your car’s computer and tell you the problem (for free). You can buy one too from amazon or the like ---
**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:** [September 1, 2014, 8:01pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/3 "2014-09-01T20:01:48Z")
Your car’s check engine light shouldn’t be affected by your coolant or oil level. If it’s working properly it indicates a problem with your car’s emissions control system. It could be caused by a buildup of gasoline vapors in the car’s fuel tank or you could have had a cylinder misfire. In my car, a 1996 Contour, I can shut the light off by opening the gas cap and releasing the vapor, then driving the car, letting it get cold, and doing this for a total of three trips with a cold engine start. That will reset the sensor if it was caused by a gas vapor pressure buildup. ---
**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:** [September 1, 2014, 8:04pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/4 "2014-09-01T20:04:38Z")
> [@dougie\_monty](#): > > I drive a '92 Ford Taurus GL with a “Check Engine” light on the instrument panel. It has been lighting up lately, and obligingly I have checked the levels of the oil and coolant. Although it literally tells you to check the engine, it doesn’t really mean that, and it’s not a warning light for oil or coolant. It means there’s something amiss in the area of fuel/ignition/emission controls. There are many possible causes for the light to come on, some serious and some trivial. > [@](#): > > It will appear when I start out and go south or west, not east or north. So I can drive somewhere to the west or south of me and the light will stay on; but returning northward or eastward, for the same distance, the light doesn’t go on at all. That’s quite odd, and interesting, if it is consistently related to direction traveled (and not, for example, to going uphill/downhill, or to temperature, or to some other aspect of the trip that simply coincides with direction). > [@](#): > > About the time this started happening, my turn signals, which have been working intermittently for months, now work all the time. There’s no direct correlation. It’s most likely coincidence, though there might possibly be an indirect correlation. ---
**Author:** ![salinqmind](https://avatars.discourse-cdn.com/v4/letter/s/3ec8ea/32.png) [@salinqmind](https://boards.straightdope.com/u/salinqmind)\ **Post date:** [September 1, 2014, 8:27pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/5 "2014-09-01T20:27:37Z")
They can’t inspect your car (for inspection, of course) if the check your engine light is on. I have a poor friend, and sometimes they will tell her to bring her old junker in, they will turn off the light somehow, and the car will then pass inspection (though the light may come back on and the main problem hasn’t been fixed.) ---
**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:** [September 1, 2014, 9:59pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/6 "2014-09-01T21:59:53Z")
In some areas (like where I live) you can’t just reset it to clear out the check engine codes and hope that it doesn’t generate a code on the way to getting inspected. To pass inspection, the engine computer has to not only have no outstanding codes in it, but it also needs to have completed all of its internal checks, which usually means driving it around for a bit with both city and highway type driving. If any of the internal checks return an incomplete status, that also fails the inspection, even though there’s no code and the check engine light will be off. ---
**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:** [September 1, 2014, 10:05pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/7 "2014-09-01T22:05:11Z")
It’s almost always the O2 sensor that needs to be replaced when the “check engine” light comes on. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 1, 2014, 10:20pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/8 "2014-09-01T22:20:43Z")
> [@John\_Mace](#): > > It’s almost always the O2 sensor that needs to be replaced when the “check engine” light comes on. Umm you got a cite for that? I will be willing to bet that the other automotive professionals on this board will disagree with you. I know I do. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 1, 2014, 10:22pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/9 "2014-09-01T22:22:08Z")
> [@engineer\_comp\_geek](#): > > In some areas (like where I live) you can’t just reset it to clear out the check engine codes and hope that it doesn’t generate a code on the way to getting inspected. To pass inspection, the engine computer has to not only have no outstanding codes in it, but it also needs to have completed all of its internal checks, which usually means driving it around for a bit with both city and highway type driving. If any of the internal checks return an incomplete status, that also fails the inspection, even though there’s no code and the check engine light will be off. A1992 car is OBD I and doesn’t do those checks. It doesn’t even have the capability to do them. ---
**Author:** ![48Willys](https://avatars.discourse-cdn.com/v4/letter/4/db5fbb/32.png) [@48Willys](https://boards.straightdope.com/u/48Willys)\ **Post date:** [September 1, 2014, 11:29pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/10 "2014-09-01T23:29:16Z")
> [@John\_Mace](#): > > It’s almost always the O2 sensor that needs to be replaced when the “check engine” light comes on. Like Rick, I want a cite for that. This, IME, is pure BS! Going to the auto parts store to get the codes read is a good idea, however do not have them reset the computer. Your mechanic will need to see these codes himself/herself. Your car just wants to go to the NE part of the country! It does not like California. HA HA! The check engine light could be caused by many things, direction traveled is not one of the common reasons for this to happen. I suspect that the terrain to the north and to the east is the same. Yet it is different terrain to the south and to the west. ---
**Author:** ![dougie\_monty](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@dougie\_monty](https://boards.straightdope.com/u/dougie_monty)\ **Post date:** [September 2, 2014, 12:13am UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/11 "2014-09-02T00:13:10Z")
> [@48Willys](#): > > Like Rick, I want a cite for that. This, IME, is pure BS! > > Going to the auto parts store to get the codes read is a good idea, however do not have them reset the computer. Your mechanic will need to see these codes himself/herself. > > Your car just wants to go to the NE part of the country! It does not like California. HA HA! > > The check engine light could be caused by many things, direction traveled is not one of the common reasons for this to happen. I suspect that the terrain to the north and to the east is the same. Yet it is different terrain to the south and to the west. Not quite. I drive to a point several miles away–southbound, and the light is on for the whole drive. I come back, northbound of course, and no light. We’ve had the care here in Gardena and the South Bay for 18 years. HA! ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [September 2, 2014, 12:56am UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/12 "2014-09-02T00:56:42Z")
> [@dougie\_monty](#): > > Not quite. I drive to a point several miles away–southbound, and the light is on for the whole drive. I come back, northbound of course, and no light. > We’ve had the care here in Gardena and the South Bay for 18 years. HA! We are neighbors, I grew up in Gardena, North Torrance now. Hard to imagine a code that is direction sensitive, I would not get them erased before your mechanic can look at them. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 2, 2014, 1:16am UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/13 "2014-09-02T01:16:56Z")
Trust me your car does not know direction. There is something different in those drives. Could be speed, number or timing of stops, load factors due to incline etc but it isn’t direction. ---
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [September 2, 2014, 3:06am UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/14 "2014-09-02T03:06:59Z")
> [@am77494](#): > > Take it to a auto store like autozone or advanced auto and they will hook up a bod scanner to your car’s computer and tell you the problem (for free). Not in California, which is where the OP is. Since _something_ happened within the past ten years or so (I’ve never found out precisely what), parts stores in this state either cannot or will not read codes or loan out scanners. Here is a [recent thread discussing it](http://boards.straightdope.com/sdmb/showthread.php?t=712482), in which no solid conclusion was reached. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 2, 2014, 3:40am UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/15 "2014-09-02T03:40:35Z")
Actually **Gary T** hit the nail squarely on the head with this post: > [@Does California law prohibit the sale & rental of auto trouble-code readers](https://boards.straightdope.com/t/does-california-law-prohibit-the-sale-rental-of-auto-trouble-code-readers/678237/24): > > Auto repair facilities in California have to be licensed as such. Performing auto repair service – which can include testing, evaluation, and advice – as a business without said license is a good way to get your tit in a wringer. California’s BAR (Bureau of Automotive Repair) tends to be tough on shops to the point of being overzealous in protecting customers’ interests. It wouldn’t surprise me if parts stores got stung with hefty fines from BAR due to disgruntled customers and decided it wasn’t worth the hassle. Auto Zone is not licensed as an Auto Repair Dealer. Per the BAR reading codes and offering advice falls under auto repair. The BAR has some serious penalties for operating an unlicensed repair shop. In addition even if a parts store were to get licensed there are a whole raft of additional rules that they would have to conform to to read codes. Bottom line it’s easier just to not offer that service. ---
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [September 2, 2014, 4:42am UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/16 "2014-09-02T04:42:18Z")
Yeah, that feels like almost certainly what happened. It’s a bit of a stretch, in my opinion, to consider loaning a code scanner to be “auto repair” (It’s a tool, just like the torque wrenches they do loan out.), but Gary’s line of reasoning is about the only thing that makes sense. As best I’ve been able to tell, all parts stores in California seemed to change their policy at about the same time - which would suggest that there was some specific game-changing BAR ruling, fine, court case, or whatever. As we discussed in that thread, though, nobody seems to be able to track down exactly what it was. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [September 2, 2014, 9:10am UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/17 "2014-09-02T09:10:13Z")
It’s one thing to scan the codes (and you may need a model specific reader to get them all) and another to interpret them. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [September 2, 2014, 8:50pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/18 "2014-09-02T20:50:19Z")
> [@brad\_d](#): > > Yeah, that feels like almost certainly what happened. > > It’s a bit of a stretch, in my opinion, to consider loaning a code scanner to be “auto repair” (It’s a tool, just like the torque wrenches they do loan out.), but Gary’s line of reasoning is about the only thing that makes sense. > > As best I’ve been able to tell, all parts stores in California seemed to change their policy at about the same time - which would suggest that there was some specific game-changing BAR ruling, fine, court case, or whatever. As we discussed in that thread, though, nobody seems to be able to track down exactly what it was. Loaning a scanner probably isn’t auto repair but when the customer asks how do I fix a P0302? The answer is squarely in the auto repair category. Picture this. Code p0302 misfire Cust: how do I fix this? AZ: spark plugs, wires, cap, and rotor should do it Cust: take my money Car still has a misfire. Cust is pissed and goes to BAR BAR to shop that will be $5,000 please and if you do this again you will go to jail. ---
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [September 2, 2014, 9:03pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/19 "2014-09-02T21:03:19Z")
> [@Rick](#): > > Loaning a scanner probably isn’t auto repair but when the customer asks how do I fix a P0302? The answer is squarely in the auto repair category. > Picture this. Code p0302 misfire > Cust: how do I fix this? > AZ: spark plugs, wires, cap, and rotor should do it > Cust: take my money > Car still has a misfire. Cust is pissed and goes to BAR > BAR to shop that will be $5,000 please and if you do this again you will go to jail. I think we’re on the same page here - that scenario is depressingly easy for me to envision. When my CEL comes on, I don’t ask the parts store guy how to address the code. Like all enlightened individuals, I get my free advice from the internet. 🙂 (And, in truth, message boards have helped me fix CEL-related issues before.) ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [September 2, 2014, 9:50pm UTC](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183/20 "2014-09-02T21:50:26Z")
> [@Rick](#): > > A1992 car is OBD I and doesn’t do those checks. It doesn’t even have the capability to do them. What does OBD mean? And OP, the Midas shop in Anaheim has a sign saying they’ll diagnose your “check engine” light for free. It’s likely you can find a place locally that will do the same for you. [Next page](https://boards.straightdope.com/t/my-cars-check-engine-light-has-the-sensor-gone-haywire/697183.md?page=2)