Having a new one (code detected) put in now and the price was no fun. So after a NOX sensor senses NOX, what does it do about it. I have no emissions test requirement. So could this have been a ‘Who Cares’ moment and money saved, or does the vehicle need a correct readout to run right? 2000 Subaru Legacy with 87K. Thank you.
What? I’m having a hard time following your post. You got a code, and replaced an oxygen sensor, correct? There is no sensor to measure nitrous oxide compounds on your car. This car has 4 oxygen sensors, so which one did you replace? Are you sure you replaced the correct one?
If it is one of the sensors before the catalytic converter, then YES, the car uses this one to fine tune it’s fuel mix. The sensors after the cat just monitor the exhaust coming from the cat.
Unless your vehicle has a diesel engine, there is no NOx sensor. Your vehicle does have a KNOCK sensor. Is that what you mean?
Tester
the NOX sensor is usually when the passenger starts giggling uncontrollably 
My first thought was that the OP must have been referring to the knock sensor, but, then when he said it was being replaced I figured he must be referring to the oxygen sensor. As already pointed out, there is no NOx sesnor.
To the OP I would point out that sensors only detect some condition to help the engine make its real-time adjustments to operate correctly, or detect the operating condition or a failure of some system or process. While a sensor itself can go bad, and they sometimes do, it requires actually doing some investigating to determine if the fault code is due to a sensor failure or some other failure. Some sensors are, due to their exposure to contaminants and/or heat, more subject to failure than most, but in general a “code” from a sensor only says something has gone wrong and points the tech in the direction necessary for diagnosis.
All vehicles need “correct readouts” from all their sensors to run right. Many sensors tell the engine’s computers how much gas to feed in, what the ignition timeing should be, and when to shift, and other sensors detect a system or component problem.
Or the back seat driver suddenly goes quiet…
Whoops, my error, that’s the CO sensor…