95 Altima O2 sensor going bad?

I checked the voltage reading from my Altima front single-wire O2 sensor. The Heynes manual says it should start from 0.1v and goes up to 0.9v as the car warms up in 2 minutes. It should then fluctuate between 0.1v and 0.9v.

My car's O2 sensor's reading starts from 0.1v and goes up to 0.6v and then starts to go down gradually to 0.1v as the car continues to warm up and stays around 0.1v. Does this mean the O2 sensor is going bad?

Thanks for your advice