Tester is correct, it probably is a defective oxygen sensor. It only costs the price of a new oxygen sensor to find out.
Just speaking for frugal me (you can use other terms, if you wish), I would use a digital multimeter ($25 at Walmart) to measure the electrical resistance (ohms) of the oxygen sensor. To do this, I would disconnect the electrical connector from the oxygen sensor, and measure the ohms between the terminals for the yellow/red wire and the black wire terminal on the O2 sensor. It should be low ohms. Compare to the voltage on the yellow/red wire of the other O2 sensor.
I would also like to know if power (12 volts) is coming in on the yellow/red wire. I would turn the ignition key to RUN, and touch the red probe to the end of the yellow/red wire, and the black probe of the multimeter to a good, clean, metal surface on the engine or transmission.
Again, compare the voltage measured (12 volts) to the other O2 sensor.