Downstream O2 sensor voltage ( aka after catalytic converter ) varies depending on many things. A car with a healthy cat, the voltage "should" stay above .5 volts when the car is warmed up. Sometimes it dips down below that mark due to lack of air flow through it, IE idle. - The traditional O2 sensor makes voltage in absence of oxygen, so a higher voltage tells the ECU that the cat is retaining oxygen molecules ( instead of by passing) so it can chemically alter the HC, NO, CO, into atmosphere friendly gasses, like CO2, and H2O. Short answer, between .5-.9 of a volt.
