O2 sensor voltage fluctuation wildly

[Giovanni]

I have a 95 geo metro. It has a 4 wire O2 sensor. My millage has not been what I think it should be so I hooked a multimeter and a air/fuel mixture gauge up. The voltage and gauge fluctuate wildly from most lean to most rich. I am sure it is not a loose connection. Does this mean the O2 sensor is bad?

Thanks