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.