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A Nissan Versa (2008 or '09) driver in the USA, running on 87-octane usually, decided to fill up from almost-empty on 89-octane based on advise that it helps clean the engine. They returned to highway driving and around 1/2-full tank, the "Service engine soon" light came on solid, not blinking.

Before knowing about the fuel mix I suggested to fill up, check the gas cap, and give it another 30 min before worrying about it. Even then, I noted that if everything else appears to be running fine, finish the highway drive and a few hundred miles later when they're back in their home town they could bring it to a mechanic to investigate. Learning about the unusual fuel mix this morning, I immediately thought, that must be the cause!

I know (in part from this post) that putting 89-octane in a car (that is made post-2000) that normally runs 87-octane isn't so bad because the engine will adjust and just run a little less efficiently. My question is: Are the adjustments an engine makes when running a higher octane mix than usual enough to cause the check engine light to come on?

Also, any suggestions on how to proceed back to maximum efficiency (and no check engine light) are appreciated. My guess is let the tank run down to almost empty again and once the E light comes on, fill it up with the usual 87-octane, tighten the gas cap, and give it a little time to settle back in to the norm.

Edit: The light went off after stopping near empty, filling up with 87, and tightening the gas cap (sounds like securing the gas cap was what did the trick). Thanks for the suggestions.

  • Check the various questions about potential advantages of higher octane fuel: there are basically none if your car is meant to use the lower-octane fuel. For the same reason, I doubt you can have issues by mixing them in any ratio. Moreover, in Europe we have 95 and 98 octane fuels. Are you sure about 87/89? – FarO Aug 23 '16 at 13:39
  • @OlafM I see the higher octane use shouldn't have any benefit or problem. Just wondering if it can cause the engine light to come on. This driver is in the USA and is sure about the 87/89 octanes. – cr0 Aug 23 '16 at 13:42
  • The real answer lies in the check engine code. What code was it? – PeteCon Aug 23 '16 at 15:27
  • @Pete how does one check the code? That's something I haven't looked up yet so you probably don't need to answer, I imagine there's good info on that in a manual or somewhere. – cr0 Aug 23 '16 at 15:32
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    Get the car to an OReilly, AutoZone or similar car parts store. They'll read the codes for free, and tell you (or the driver) what's wrong. They're not mechanics, so they sometimes get it wrong, but just the code number will help us here (i.e, don't come back with 'They said it was a misfire' - give us something like P0302). Sometimes there'll be more than one code - we need them in order of display, not numerical order, because the pattern can be important. – PeteCon Aug 23 '16 at 15:43
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Mixing octanes in an upward-direction is fine. But, not completely tightening the gas cap could cause that. If you want to know exactly why the CEL came on, just take the vehicle to a retail car-parts store, such as Autozone or NAPA. Most of them will pull trouble codes for free.

(You mentioned gas cap above, but your wording in that sentence is confusing and so I'm not certain why you mentioned it. If it was the gas cap, the CEL will eventually turn back off but it will take a little bit of driving.)

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Using a higher octane fuel should not have caused the SES light to turn on.

The only fill-up-related thing I can think of which would cause the SES to turn on is if there is a bad batch of fuel (something like water). In such a scenario, the SES will be triggered due to the engine knocking. This would be typified by engine sputtering, and is usually accompanied by loss of power if the engine management decides to go into limp mode.

As none of these symptoms were reported, it is difficult to gauge the real cause though. The surefire way to tell what caused the SES is to query the engine computer via a scan tool.

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    One other fill up related CEL: leaving the gas cap off. – Pᴀᴜʟsᴛᴇʀ2 Aug 23 '16 at 15:26
  • With none of those hazardous symptoms reported, think it's OK to complete a few hundred miles back to home base and known mechanics, or better to get it checked ASAP? Of course it ends up being the driver's decision and it can be a major one. I'm thinking if the SES light is blinking, that's a sign to stop at a mechanic ASAP (and be very careful about driving at all en route). Since the light is solid, it's something to check as soon as convenient (which in this case is in another ~300 miles). I'll look into checking SES code also. – cr0 Aug 23 '16 at 15:38
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    If the light is solid, and the car hasn't lost performance, it could well be an O2 sensor code, which would be fine to drive a few hundred miles with. But my previous advice to pull over at an Oreilly or Autozone etc still applies. – PeteCon Aug 23 '16 at 15:45
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    Like Pete mentioned possible O2 sensor. I've seen bad gasoline contaminate and kill Oxygen sensors. There have been lawsuits over that. It's an expensive repair (if you don't do the labor yourself. ) – zipzit Aug 23 '16 at 16:22

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