I thought 85 octane would be fine as well then I got a check engine light so I got it tested and it came up with a pre cat oxygen sensor failure. I replaced the sensor and the check engine light stayed on. I got it tested again and they said bad pre cat oxygen sensor again. When I told them I had just replaced that sensor they asked what gas I was running to which I replied 85 octane. They (Autozone) indicated that running with lower than recommended octane can cause a false reading from the oxygen sensor. They recommended running a can of fuel system cleaner and changing to 87 octane. I did that and after about 400 miles the check engine light went out for about another 400 miles and now it is back on again.

What would you all do at this point?