PCMs and ECUs for that matter have internal voltage divider networks on all O2 sensor inputs which hold those lines at 450 mV DC (stoich) in the absence of a signal from a sensor or simulator. We can all verify this by watching our O2 voltages during key on and cold start. The input value will hover very close to 0.45 V DC until the sensor heats up and begins to "pull" the voltage higher and lower according to richer and leaner conditions.