Q: check engine light on. dealer said too high alcohol content on 2001 Toyota Avalon

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The engine light & the 2 VSC lights came on. Dealer said a high alcohol content & running too lean. I ran the tank near empty & filled with SHELL premium, was OK. Refilled with regular and before too long same thing. This routine is now on the third round. This last time we drove about 100 mi and the lights came on again. any suggestions?
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They didn't tell me the code #. After they ran the first diagnosis,they asked me if I had filled up with E85 fuel, which I didn't. They said the computer gave a too lean a mixture reading. Supposedly everything else was up to specs. I talked to a local mechanic and he thought maybe the sensors that pick up this reading are not doing the job.
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