found this relating to toyota titania sensors The A/F sensor signal voltage varies according to the amount of oxygen sensors in the exhaust stream Unlike the signal voltage from a heated oxygen sensors (HO2S), the A/F sensors signal voltage increases as the air-fuel mixture leans and decreases as the air-fuel mixture richens. The signal voltage ranges from 0.48 to 0.80 volts! While the vehicle runs in closed loop mode, the ECU uses the A/F sensor signal to lengthen or shorten the fuel injector pulse width until attaining a stoichiometric air-fuel mixture Besides, on Engines with these Sensors voltage that corresponds with a specific A/F ratio is above (to 4.0 Volts).