Your O2 sensor isn't going to cause much other then slightly poorer mileage than normal. The average mileage I've seen would be about 18MPG with mixed highway and city and a slightly heavy foot on occasion. You can unplug the O2 sensor and should see little change in the operation of the vehicle. If there is a significant change then there is a problem with a more critical trim sensor somewhere else in the system (such as the AFM).
The code could be the result of low voltage due to the cold temp and drain from repeated cranking... test the battery, the alternator and test the ignitor.
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