I am aware of the fact that a dead rear O2 sensor would cause the engine to run rich and give me crappy gas mileage, but would it also cause crappy performance overall? The car does feel a slightly sluggishier than before...

 

The rear O2 sensor does not control the Air/fuel ratio. I also won't cause bad performance or bad mileage.

 

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Gah I give up. I have no idea whats causing the sluggishness and the bad gas mileage I get no codes other than the dead O2 sensor; car has only barely 89K miles on it - yet when I accelerate there are slight but noticeable rapid vibrations, there is the crack-in-the-window-and-air-getting-in sound when its WOT and past 4K RPM, there is the crappier gas mileage (avg of 22.7 MPG down from like 25 i've been getting), and of course the sluggishness that I feel...

I am at a lost

 

What else have you done so far to try to fix the problem?

Maybe an air leak in the air intake system somewhere?
- leaking intake manifold gasket
- dirty MAF sensor
- dirty throttle body
- leaking throttle body gasket

possibly an ignition coil going bad or bad knock sensor..

just throwing out ideas..

 

I'm not 100% about this but someone on the forum told me before that when sensors need to be replaced, the ecu gets set to a lower standard to protect the car. At least this was the case with my knock sensor. My car had the same symptoms as you but as soon as I replaced it and resest the ecu, it was back to normal.

 

thats only a knock sensor....the worst that will happen with a bad FRONT o2 would be running rich...rich is default mode when they die....a bad REAR o2 will just cause the CEL ligth to come on because it cant properly monitor the catalyst

 

Both the maf and the throttle body were cleaned recently when I did the clutch swap. I don't know about the throttle body and intake gaskets but I have no real way of testing. I don't know about the Knock sensor because I only can read the O2 code... I'll try and test the knock sensor with a multimeter again... last time I did it, the readings were fine.

 

since you have a 99, the coils are a probable culprit for your lower mpg

 

even with no codes? and i suppose if they do go bad the only thing i can do really is to measure them - which is a PITA

 

I suppose I can risk getting used 66K mile hanshin coils and some new plugs and see if that would do the trick...

 

Yeah, the vibrations at certain idle could be the flywheel but I doubt it would vibrate with accleration though, maybe... Everybody has been on an EGR kick lately but that wouldn't affect this stuff that much I don't think. I'd try cleaning the MAF, IACV, TB and even intake manifold. Then maybe EGR for kicks. Make sure all vacume lines are ok and all bolts are torqued right and check all gaskets too.

edit: Your only pulling an O2 sensor code and nothing else?

 

yup, o2 codes and thats bout it

 

Well your best bet is to buy the new O2 sensor I have for sale:

http://forums.maxima.org/showthread.php?t=506441

 

Yes change your rear O2 sensor and the problem should go away i had the same problem and getting crappy mileage and was hesitant to replace the O2 reading in the forums but it was completely reversed there has something else has to do with rear O2 then just the emmission.
Just my 2 cents.

 

I already bought and received a Denso for $40 ^_^ Thanks anyway though!

 

Your .02 isn't even worth 1/2 of that.

 

I refuse to believe that the rear o2 affect nothing but emissions when it fails. If it is unable to monitor the difference between front and rear readings, it is GOING TO affect the mileage. They are what control the amount of air/fuel so what makes you think that they wont have an adverse affect on mileage? Replace the faulty part that the car is telling you to replace. Reset your ecm. Drive. You should be all set unless you have other problems.
Also, when an o2 sensor code is thrown, it USUALLY sets a knock sensor code ALSO. Weird if it isnt on yours.. but replace the o2 before spending money on a knock. If after resetting, you throw JUST a knock code, THEN replace it. A knock code usually accompanies other codes (o2, emission, egr) and does NOT necessarilly indicate fault in the knock sensor unless the code is thrown alone.
BTW, Im lucky to get 22 mpg on my max.. LoL! Im usually around 21-22mpg.
Good luck..
DJ

 

You must not have ever read any part of the EC portion of the factorty service manual.





First things first.

The primary o2 sensors are different than the o2 sensor in the rear.

Not only physically different, but also placed in a different area of the exhaust stream.

What this means, primary o2 sensors are placed upstream in the exhaust system prior to any catalytic convertors.

Reason for this is because they (PO2's) send feedback to the ECU, and the catalytic convertors will skew the a/f reading, therefore the ECU will not get the correct a/f reading, in turn giving a faulty signal and in the end, affect gas mileage.

The rear o2 sensor(RO2) is also different in construction when compared to the PO2's.

It also serves a different purpose as well. It monitors the health of the main catalytic convertor.

And besides, after the engine has reached even the slightest bit of load (ECU goes into open loop), ALL of the o2 sensors are ignored, and the ECU uses readings from the MAF and CPS for reference.

 

Drove with bad REAR O2 for over a year and it does NOT affect gas mileage (I reset my trip meter on every fill up and keep track of it for the sake of curbing my driving habit. If I don’t get ~180-200 miles when the gas gauge is in the middle then I know I am driving too hard). Only gives a CEL light. Annual inspection came around so I had to get a new one. Same utilization of gas (I also reset the ECU a few times). More than likely it is a bad knock sensor as people have indicated that a 'going bad' knock sensor will not throw a code.

http://www.helpforcars.net/Ownership...k%20Sensor.htm

 

I agree with the coils theory. I started getting bad gas mileage ~75k-80k and I got a bad coil code at ~85k, I replaced all coils last week, so far it looks like my tank of gas is giving me a longer run....still awaiting next fill.
Still sluggish under 3k rpm??? but then I also have a rear O2 sensor problem that
I should fix someday....
Old coils mileage 20-22, estimate new coil mileage 23-24.
If I would have known better I would have replaced the coils on the 60k.