FYI: My car has gone thru several O2 sensors in it's life (4). In the latest episode, the Check Engine light came on again and rather than scheduling a fix at the dealer, I decided to live with it a while in this condition. Yes the mileage went down. But when I refueled it with E0 gas available at the local Sunoco station (called 'recreational fuel'), the light went out after about 100 miles. I usually buy gas at the local Costco, but all this jogged my memory. Each time the O2 sensor went on vacation, I had filled up from another station out of town. I believe the condition of the gas at that station out in Webberville which has E15 is the cause of the engine control issue. The O2 sensor gets 'polluted' from the E15. When I refuel at Costco, the light eventually goes out.