The relay is just power supply. if the system works the relay is working. The fact that you had a 50% duty-cycle means that its working. (any reading means its working).
The fact that you get the duty cycle means you should retry the current. The current should be zero at 50% duty cycle though. Fifty percenet duty cycle should go away as soon as the O2 sensor heats up. A constant duty cycle of 50% indicates a code for an open O2 sensor circuit.
To verify the current is working look for a 20ma current with key on engine off.
Bosch Master, ASE Master, L1
33 years MB technician