By on September 15, 2017


engine lights

Looongtime TTAC commentator PrincipalDan writes:

I recently had an experience with CEL (check engine light) and VSC light (vehicle skid control) that left me scratching my head: I was on my way into Gallup (30 mile drive) and as I was getting up to speed on the highway the CEL and VSC came on at the same time. I know the CEL can be triggered by a dozen different things but seeing a constantly lit VSC was a new experience.

The vehicle (2010 Toyota Highlander) drove perfectly normally and I went on to my destination. I consulted the forums and they said the CEL would AUTOMATICALLY trigger the VSC light and that the VSC would basically be shut off by the triggering of a CEL. Dafuq!?!?!? Really Toyota?

A CEL (which could be triggered by something like an improperly tightened gas cap) will shut down one of the key components of the safety systems of the car? What’s the logic behind this? More lights means the average American is more likely to go to the dealer and get it checked out? Do all of the manufactures do this now?

BTW, my lights were triggered by a bad gas cap. Couldn’t get it to “click” anymore so I replaced — lights went away and problem solved.

Sajeev answers:

While it’s unfortunate the problem was the ubiquitous “gas cap fail,” the reality is most active handling systems tweak engine performance as part of their functionality. Be it ignition timing retardation or reining-in the electronic throttle, VSC needs an error-free engine computer. So why isn’t the car “smart” enough to leave the VSC on for seemingly irrelevant CEL warnings?

I suspect corporate lawyers (with undergrad engineering degrees) would beg to differ, risk aversion in mind. Why risk it when you might be wrong?

BONUS: A Piston Slap Nugget of Wisdom!

Think about this from another angle: consider the defense mechanism of the Puffer Fish. Now imagine a vehicle computer illuminating relevant idiot lights to save its bacon. It’s done in hopes the driver addresses the problem before it worsens, respecting your wallet, the manufacturer’s wallet, and/or our air quality. And lights can flash on/off when things are going sideways quickly. When things are hemorrhaging (literally), it’ll shut down the engine because you aren’t checking the oil pressure/level every second. That’s a smart defense strategy!

[Image: Shutterstock user gdainti]

Send your queries to [email protected]. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice. 

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40 Comments on “Piston Slap: VSC Light and the Puffer Fish Effect?...”

  • avatar
    Stanley Steamer

    On Subarus, a CEL will shut down the cruise control system. That makes more sense as it’s annoying enough to get your attention without trying to kill you.

    • 0 avatar

      Frustrating on old cars when you know what the issue is…

      IE- I have CEL issues on 3 cars, one because of aftermarket modifications and 1 because of aftermarket modifications, and then the last one because of aftermarket modifications.

      On one of them, my truck, my tune “turns off” the CEL, but doesn’t turn off the truck thinking there’s a CEL. On one of them, I installed some resisters to fake the computer into thinking there’s no CEL need, but it still triggers it sometimes, and one is frustrating as all get out- taken it to multiple shops, replaced multiple parts, and I still get the CEL related to an aftermarket exhaust system (I replaced a problemmatic header-cat with a main cat thats the same size as the total cat system, so its still a 300 cel system like the factory, but I moved the precats out of the header where they can get sucked into the motor- common mod).

      I just can’t wait for the next “me” to be dealing with 2018 cars the way that I’m dealing with 2001-2008 cars… Cars are getting more and more challenging to modify!

      • 0 avatar

        What is the DTC code? How did you change the exhaust configuration?

      • 0 avatar
        Felix Hoenikker

        1. How does a cat get “sucked into the engine” when the exhaust system is always under positive pressure except in the case where an exhaust valve is stuck open? Then you have bigger problems.
        2. By removing the pre cats you have altered the time it takes the cat to light off the unburned hydrocarbons during engine warm up which is the dirtiest time of the operation. The powertrain control module will detect this via the difference in oxygen concentrations in the exhaust and trigger a CEL because it thinks the catalyst is spent.
        3. Do you live in a non emission inspection state because the mods you describe appear to be violations of Federal laws?

        • 0 avatar


          Assuming the exhaust is always under positive pressure is an easily understood fallacy.

          Under certain load cases there is enough turbulence and acoustic reversion that loose particles could be pulled upstream. Put into a high-RPM engine braking scenario, especially one at the end of a WOT acceleration, and pressure in the exhaust could easily exceed cylinder pressure prior to the exhaust valve closing. It’s not impossible for exhaust to be pulled up the intake during the valve overlap period; it would be easy for a failed precat to be partially ingested if it were close enough to the exhaust valve.

          • 0 avatar

            Particularly in vehicles with VVT where it replaces EGR (without the need for extra lines, valve etc). I recall that Nissan had an issue with pieces of damaged cat being sucked back into cylinder and resulting damage to the engine.

        • 0 avatar

          I think some years of the Taurus SHO had a similar problem, where the ceramic matrix inside the catalytic converters would crack, then loose particles could get sucked back up the headers and into the cylinder heads, where they caused major damage.

      • 0 avatar

        On getting rid of a cat code from an aftermarket exhaust, try a simple “spark plug fouler” spacer/adapter for the after-cat O2 sensor. It almost always gets rid of it and the rest of your emissions systems works properly.

    • 0 avatar

      My co-worker, who does a lengthy commute, ran into this problem with his old Subaru Outback. As I heard it the head gasket was leaking, which caused the cat to fail. He had the head gasket fixed but didn’t want to throw any more money into a 200k+ mile car. So now he makes his commute without cruise control.

      Because of this one issue he refuses to buy Subaru again, even though he loves the AWD during Michigan’s wonderful winters.

    • 0 avatar

      What year? My 97 Outback has been throwing an O2 sensor CEL for about 10 years now, cruise still works just fine.

  • avatar

    This post is related to the earlier one about High Tech in premium vehicles. As modern vehicles become less “mechanical” and more “computerized” they become more fragile and in general more difficult & expensive to fix. Interconnecting the various “modules” only exacerbates the problem.

    This is not to say that traction control is not a good thing. And a CEL is also a good thing, especially with the dearth of engine gauges and the general lack of attentiveness to one’s vehicle common today.
    But remember, they are called idiot lights for a reason.

  • avatar

    Lets pretend you forgot to put your gas cap on. Triggers the CEL, I get that, but I think my car has a separate light for that. Anyway, you gas cap is off, gasoline vapors are now polluting the air. Computer tuns off a major safety feature. You pollute, you die! What are you people not getting!

    • 0 avatar

      Some vehicles do have a check gas cap light but it will only illuminate when there is a massive evap leak caused by the gas cap not being present but will set a generic evap code when it is due to a cap that doesn’t create a perfect seal. Some also look for a refueling event as part of the decision to turn on the check gas cap light or set a generic evap code.

      Just because the computer showed the check VSC light does not mean that it was disabled.

      • 0 avatar

        my Ranger has a gas cap light. IIRC it will light if there’s a newly-detected EVAP leak *AND* the measured fuel level has increased more than a certain amount since the last key cycle. if it’s still there after subsequent key cycles it’ll just go to a Check Engine light.

        • 0 avatar

          Yup Ford is one of the mfgs that has implemented a gas cap check routine. IT senses the refueling event which makes it run a quick large leak test, even though the conditions are not present to run the full evap test. Run it long enough with the gas cap check light and then it will also light the SES light. They of course are assuming that a person will actually check the cap when they see that light and that it did not in fact fix leak it is sensing.

          • 0 avatar

            Fords don’t have a separate gas cap anymore…probably why they implemented that routine.

            Failure means a trip to the dealer…no way for the owner to check it.

          • 0 avatar

            The check gas cap indicator was on Fords well before they went to the capless filler system.

  • avatar

    Meh; I think it’s more: “It’s a lot of work to determine exactly which engine issues might affect the VSC system, so we’ll just turn it off for any of them and call it a day.”

    • 0 avatar


      All day, err day.

    • 0 avatar

      Being Toyota, I’d say this is highly likely. Plus as was mentioned in the article it may be their way to scare people into paying attention to the CEL when there are no driveablity concerns caused by the particular failure.

      • 0 avatar

        Actually, on my 03 4Runner, it really does disable vsc and traction control. A ‘lean condition’ CEL triggered all 3 dash lights. I was ignoring it and forgot that it affected those systems. I was rudely reminded when the back end started to step out on me accelerating out of a turn on wet pavement. I ‘fixed’ it the next day …. just pulled the battery cables, and it cleared.

      • 0 avatar

        Why not disable the air bags, unlock all the seat belts, and protrude a spike from the steering wheel? That will get your attention!

    • 0 avatar

      While this is absolutely true, allow me to add a bit of light and shade.

      Jack wrote about this awhile back, but once upon a time computer code for embedded systems like this tended to be written by a small number of very talented engineer employees, who likely worked massive amounts of overtime to create the entire system because they were passionate about their work (and didn’t feel like they had anything better to do).

      This has mostly transitioned to large numbers of semiskilled contract coders who are responsible for and have knowledge of only one subsystem within a much larger whole, who likely still work massive amounts of overtime because that’s what’s expected in the “biz”.

      On a practical level, going from hundreds of lines of assembly code to hundreds of millions of lines of Java code pretty much requires more hands to be involved. But the shift to assembly line code means that very few people within the project have a thorough understanding of how all the systems interrelate.

      Plus everything these days gets shoved through an API that was set up 10 years ago that doesn’t support any kind of intelligent error reporting so even if you wanted to write something better you’re hamstrung.

      So it may not even be possible to generate a reliable matrix of failure states to determine whether a system would be affected. And therefore, yeah, go the CYA route.

  • avatar

    Perhaps my greater concern is that the CEL has become the true “idiot light” because so many stinking things are connected to it.

    I’d rather have warning messages that say “O2 sensor”, “gas cap”, “misfire cyl #3” etc. But I also acknowledge that I am outside the realm of the general public due to actually having a modicum of automotive knowledge.

    • 0 avatar

      Many cars do have a gas cap warning but as I mentioned above that is for a massive leak because you didn’t install the cap, your bad cap that creates a small leak will not be distinguishable from any other small evap leak. Those check gas cap lights exist because the general public doesn’t have a clue and the dealerships were seeing too many cars coming in for a gas cap that wasn’t on or fully tightened.

    • 0 avatar

      except it’s not that simple. OBD fault codes tell you *where* a problem is, they don’t tell you *what* the problem is. codes do the same thing as if you went to the doctor and said “my shoulder hurts.” Your MD still has to do diagnostic tests to figure out if your shoulder hurts because of a torn rotator cuff, arthritis, neuropathy, etc.

      why do you think so many parts store chains are glad to read codes for free? because they’re counting on you to spend a lot of money shotgunning parts onto your car in vain attempts to fix a problem you can’t properly diagnose.

      • 0 avatar

        Well put Jim. At the same time, it’s still nice to have the OBD2 codes to help start figuring things out, or maybe determine the relative severity of the issue (say it happens during a trip). I love my ScanGauge II and it sits permanently reading off things for me on my 4Runner.

        • 0 avatar

          The light already tells you the severity of the problem. If it is on solid it is a minor problem and the car *may* be polluting more than it is supposed to. If on the other hand the light is flashing there is a serious problem, and continuing to drive it for long periods could cause additional damage. It is all there in the owner’s manual for most vehicles.

    • 0 avatar

      My acura will toss a code on the nav display. It will also bring up the nearest dealer with “navigate to dealer”.

  • avatar
    SCE to AUX

    My 05 Scion (Toyota product) would throw 3 lights in rapid sequence: skid control, traction control, and CEL. The car would drive perfectly normal afterward, and condition would clear after a while.

    My OBD-II computer didn’t find any error codes. A close examination of the engine data revealed that the downstream O2 sensor was flatlined at 75 mV. O2 sensor voltage should change in real time.

    The car drove normally because the downstream O2 sensor isn’t used for engine control; it merely monitors the performance of the catalytic converter.

    So I installed a new downstream O2 sensor, and the condition never happened again.

    In 7 years and 70k miles, this was the only thing that ever failed on the car, besides a window switch the dealer replaced when it was 3 months old.

  • avatar

    My 2013 TSX tells me to tighten the gas cap. Gee what a concept, it actually understands whats wrong. Toyota is being lazy and/or cheap. Similar to the crazy way there Manual modes indicates gear selection. See Jack’s review of the Camry a few posts down.

    • 0 avatar

      No it does not really understand what is wrong. It just knows that it has a massive evap leak and tells you to try the easiest and what was found to be the most common cause of the problem.

      As soon as OBD-II was required lots of dealerships had high numbers of brand new cars with CELs that the dealers found loose caps on.

      So at the request of dealers, who were wasting a ton of money tightening gas caps for free, some mfgs implemented the separate gas cap light. Some even went so far as to have a gas cap check, that is separate from the standard evap test that is run when a refueling event is sensed.

      Apparently Toyota dealers were not bothered by tightening gas caps and many probably charged customers for that. So by making the car Christmas tree at any fault they end up driving more business to the dealership who now have a good excuse to charge that 1 hr min diagnostic time.

  • avatar

    And to add to that, our A6 acts like the world is going to end when you door is open and the car is in gear. It thinks you might be walking away without putting it in park. One of the many issues with the push button start on modern cars cause my no key interlock for this. I get what it is trying to do and it is very valid if you are indeed walking away. But surely it can figure out the difference between that and the car being driven with the door closed on the first click but not closed tight. If you are sitting in the seat with the seat belt fastened and pressing the accelerator and turning the steering wheel I think at this point there pretty much better be someone their. So maybe being helpful and at some point in all the flashing and blaring by actually tell me what the problem is. At no point does it suggest you may need to close the driver’s door. After you figure it out once sure you know what to do but the first time it was completely confusing.

  • avatar

    It’s one of the stupid things about Toyotas – and MIL code will also turn on the TRAC and VSC lights. Our ’08 Sienna recently experienced it, with a P0171 (lean mixture on Bank 1 and Bank 2). You’d normally think MAF sensor (I cleaned it), or O2 sensors, but it turned out to be low fuel pressure (28psi, where it should normally be 42psi). It didn’t affect the way the van drove, or cause any damage. It just needed a fuel pump.

  • avatar

    Oh, the stabilitrak light. Thanks GM.

    This can be set off by bad reluctor rings, they rust and you have to replace wheel bearing assembly. This tosses no CE lights. I hope the exec who saved a penny by spacing cheap steel for the magnetic ring buys his creation one day. Bearings were OK…just not the ring. grrr. “Ding…Service Stabilitrak””

    This charming attention note is also set off when the brushes in your throttle body begin to wear and send odd voltages back to the brain. Replace Throttle Body. You get a related CE code, but it’s the TB, not the pedal. Genuine GM Parts.

    If it gets bad enough, you also get “Low Engine Power” which will get you 40 mph, inconvenient if it goes off on a highway-with no medians-uphill-. Then, shut off, reset, and drives OK until the next “Ding”.

    Overall, though I’ve found “the CE light” to be helpful, even if inconvenient at inspection time. I do usually spot a missing plug/coil before the light goes off, not that I’d ever know which one without a code.

  • avatar

    My Nissan 350Z does the same thing. My cam position sensor is flaky, when it can’t figure out what is going on the car stumbles like its going to stall… then I get 3 lights: SES (service engine soon), VDC (vehicle dynamic control) and SLIP (traction loss). The SES is easily reset by my OBD-II Bluetooth interface but the VDC and SLIP lights require a shut off and restart of the car. The OBD-II code confirmed the cam position sensor circuit failed. Seems to happen randomly but the forums have confirmed at my mileage (@ 70K now) these sensors tend to die. Once they finally go you can’t start the car! As mentioned the reason an ignition problem trips the anti-skid/traction control is the car will adjust the throttle (drive by wire) to ensure you don’t spin out. Seems like overkill but all these things are tied together and clearly designed to avoid cascade failure type situation where one system’s failure renders multiple systems useless. And the only way to inform the driver is a Christmas tree light show in the dash.

  • avatar

    My guess is 99.9% of the reason this happens is fear of litigation and a way for the car companies to CYA.

    Which is unfortunate because I would argue it makes cars less safe. But there are all sorts of costs to living in an overly litigious society.

  • avatar

    Had the same problem with my Lexus IS, gas cap tightening didn’t help always, had to put some [euro] 98 octane fuel in and drive on a highway to get rid of the lights and text on info display. I thought some sensor was the problem.

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