Irving, automotive petrol lambda sensors take a direct output from the sensing element so they output a varying voltage.

Post cat sensors still change occasionally, although I'm not sure how much the ECU monitors their performance, so even by applying a steady voltage faults may still get set.

Another snag you may have to allow for is some ECUs control and monitor the Heater section, so you may need to add a pretty beefy resistor to apply a dummy load to avoid any faults relating to the heater circuit.