All 3 of those things are currently wrong with my car.

First of all, how important is it to replace the rear 02 sensor? From what I have gathered all it does is make sure you pass emissions tests. Is that all it does?

I also got the code for a bad knock sensor. I understand it is pretty important to replace that. Anyone have an idea on how much it will cost to have that done at a dealership?

My third question is could either a bad knock sensor or a rear 02 sensor be causing a rattle in my timing chain area? I've heard an engine flush will fix that rattle, or that I need to replace my timing chain tensioner, or if worst comes to worst, the actual timing chains. I've also read that using the wrong kind of oil or a low octane gas will cause the rattle and that using the correct oil or gas will correct it.

Any help to my problems are greatly appreciated.

Thanks!

 

First reset your ECU and give it a few days to see if the codes come back. If O2 and KS codes come back, replace the O2 sensor only, reset ECU and wait to see if the KS code comes back (it seems that a bad O2 can cause KS code too).
Rattle in the timing chain area = replace timing chain tensioner.

 

the o2 sensor doesnt really do much, but you wanna fix it anyway... www.oxygensensors.com its 65 shipped for an oem sensor...as for the knock sensor, look on ebay, or on here, they, as well as the 02, are not that hard to change, and the dealer wants 135 for each sensor

 

Most dealerships will quote you $400+ to change out a knock sensor. They justify this by telling you that they have to pull the intake manifold off to replace the sensor. The Haynes manual even tells you to pull the manifold.

Stepping back into reality, you don't have to pull the manifold to change the KS. You can do it yourself in 15 minutes or less with common tools.

If you decide to change the KS yourself, look here for instructions: http://www.skippynet.com/maxima/knocksensor.htm

 

So, if the bad rear 02 sensor is causing the bad knock sensor, is the knock sensor actually acting bad (loss of power) or is it showing up as bad while everything is fine?

I've got an appointment with a nissan dealership tomorrow to check out my timing chain rattle. If it is the TSB problem he told me it would be $100 labor plus the price of parts. Anyone heard of anything else to fix my rattle?