Three days after changing the knock sensor I got 0707 code - rear O2 sensor.
Is that usual? Is there any connexion between these two sensors?

How critical is rear O2 sensor?

 

No connection between the Knock sensor and rear O2 sensor. You can replace it yourself. Very easy. Also this one won't effect your gas mileage.

 

But what is its purpose? I have to buy absolutely a Nissan part?

 

It's purpose is to monitor the effectiveness of the catalytic converter - it doesn't affect performance (unless the other O2 sensors are not working).

Just buy one from www.oxygensensors.com, any brand will work. I recommend buying one that is plug and play rather than universal (where you have to splice in the old connector). Either will work - I just stated my preference.

 

could be any problem if i'll change it later (few mounts) ?

 

Mine's been on (0707) for over a year ... no problems besides a CEL/SES light.

 

As NMex said - no problems leaving it as is for a while... as long as you don't need to go for emission tests or whatever, most states won't let you pass with a CEL code stored.

 

i have 0707 -- but it takes ~ a week to showup after reset
i just got inspected

reset, drove around ~ 7 days and it still hadnt come on so i quickly went to get my inspection... it came back on the next day after getting my shiny new sticker