# Oxygen Sensor



## Renegade757 (Jan 3, 2011)

Greetings,

Recently, my check engine light came about and I took it to the mechanic. He told me that it was a faulty oxygen sensor but did not specify which one(s). I would like to know how many sensors does the Sentra have and there locations. My vehicle would be a 95' Sentra GLE.


----------



## jdg (Aug 27, 2009)

If that mechanic says "oh it's an oxygen sensor" and doesn't give you a code or any reason why "it's an oxygen sensor"....RUN, don't walk, away from him. Some of those clowns think everything is an oxygen sensor.
Did he clear the code? If not, get it cleared, drive it and see what happens. Could've been a bad day for the O2 sensor or the ECU or both. Could happen.
When it does come back, read this:
http://www.nissanforums.com/b14-95-99-chassis/84062-how-manually-pull-define-ecu-error.html
Then read the applicable one of these:
Nissan Service Manuals


----------



## Renegade757 (Jan 3, 2011)

He inserted the scanner into the port and read two codes. One of them he said was the oxygen sensor and the second was the speedometer. He told me the speedometer code was irrelevant and he cleared it. He also cleared out the oxygen sensor code and told me that if it came back it would be the sensor. To my expectations, the check engine light turned on again.


----------



## smj999smj (Jan 1, 2006)

You have two oxygen sensors, one "upstream" or "front" oxygen sensor that screws into the exhaust manifold and is easily visible under the hood. The "downstream" or "rear" oxygen sensor is strictly a monitoring sensor and, unlike the front sensor, has no affect on drivability; it is used by the ECM to test catalyst efficiency. Location depends on your Sentra's emissions certification. Federal models have the rear sensor screw into the back of the catalytic converter. California certified models have the rear sensor screw into the exaust pipe, near the right side axle.


----------



## Renegade757 (Jan 3, 2011)

If the downstream sensor has no effect on vehicle performance besides emissions, I think that's my guy. I'm assuming this since I have not yet experienced abnormal performance. I drive a manual transmission, will that change the type of sensor? Or are all sensors the same?


----------



## smj999smj (Jan 1, 2006)

Just because you're not experiencing drivability issues doesn't necessarily mean it's the rear sensor. You need to get the code, or codes. Manual trans makes no differance in the type of sensor, put possibly the part number on some models. Most oxygen sensors work similarly, differing mainly with the length of the harness and connector type.


----------



## jdg (Aug 27, 2009)

I'm fairly sure the front is a 3 wire sensor and the rear, a 4 wire sensor. Same functionality, different hookup.
And it's quite possible that with a dead front O2 sensor, the only thing you might see is a slight drop in fuel mileage, or maybe none at all. All other things being equal, the car could run just fine without any O2 sensors. I'd bet your cat would get plugged up eventually, but that would be about it...maybe...MAYBE burn up the plugs just a tad bit quicker, but I'd doubt that too.

(I got beat to the punch!  )


----------



## Renegade757 (Jan 3, 2011)

I'm pretty sure it's the downstream sensor. The upstream sensor got fixed a year ago. My car wouldn't start and if it did start, it would stall out. I took it to the mechanic and he said was the oxygen sensor.


----------



## smj999smj (Jan 1, 2006)

Actually, the front oxygen sensor is very important and the main input for the ECM to determine the richness and leaness of the mixture.


----------



## Renegade757 (Jan 3, 2011)

Which sensor should I change? Or should I buy a scanner and post the code it reads?


----------



## smj999smj (Jan 1, 2006)

Most auto parts stores will retrieve the code for no cost (your Sentra is OBD II compliant). Or you can retrieve the code yourself using the diagnostic mode screw and LEDs on the ECM (check back on post #2 for the procedure).


----------



## jdg (Aug 27, 2009)

smj999smj said:


> Actually, the front oxygen sensor is very important and the main input for the ECM to determine the richness and leaness of the mixture.


Well sure, I get that.
What I'm saying is, that all other things being equal, a well tuned engine (with an ECU that has recently updated Short Term and Long Term trim values and updated 'tables') is just as capable of running well 'open-loop' as it is 'closed loop', just won't run nearly as efficient or clean in 'open-loop'.
Now, if the sensor was partially out and giving crappy readings (i.e. full lean, full rich, slow swings, whatever), sure, that'll cause the ECU to make it run like crap (garbage in, garbage out). But if that front O2 sensor is completely dead (i.e. disconnected), the ECU would ignore it altogether and go with the 'learned values', and assuming those 'learned values' had been previously trimmed out to good values, and the battery hadn't been disconnected or the codes cleared, which resets those values back to factory spec's (which the engine isn't factory spec anymore since it's likely got a BUNCH of miles on it), the engine should run just fine.

If the ECU is compensating for something else (i.e. vacuum leak, leaky injector, plugged up injector, low fuel pressure, and so on and so on), ya, forcing it into open loop would probably cause it to run like crap, if at all.

But ya...overall, pull the codes, swap that sensor...'nuff said, call 'er good (hopefully).
Easy job...unless you don't have an O2 sensor socket (which cost about $20 last time I checked) and/or the thing is rusted itself in there.

For grins, I'm going to go out tomorrow and disconnect my front O2 sensor on my '97 Sentra and see how it runs. I know my '98 200SX and my '01 Dodge pickup runs fine with the front O2 sensor disconnected (both times my own dumbass mistake). Obviously it pops a light that's gotta get reset, but it runs ok.


----------



## lukesSX (Feb 17, 2010)

Take it to Autozone and have them read the codes for free or call up that mechanic and ask which one it was.


----------



## Renegade757 (Jan 3, 2011)

**Update**

The problem was the wires on the sensor were stripped. It was very corroded and filthy. Thanks for the help gents.


----------



## jdg (Aug 27, 2009)

Well, that'll do it for ya.
Any chance the same thing is the issue with your other two codes in the other post? Maybe the same wire harness...maybe the same dirt/crud?


----------



## Renegade757 (Jan 3, 2011)

I hope it is, just a simple replacement of corroded parts.


----------



## smj999smj (Jan 1, 2006)

If the circuit for the O2 sensor "opens," it doesn't use learned values but the ECM goes into O2 sensor fail safe which is 0.33 volt...at least on a Nissan, anyway.


----------



## jdg (Aug 27, 2009)

BTW, I went on a 120 mile trip this morning and decided to try out my theory from a few posts ago.
Drove the 1st 120 miles with everything normal, then on the way back, after about 20 miles, after the engine was fully warmed up, I got out and disconnected the front O2 sensor with the engine still running. Obviously I got a light almost right away. But more importantly, I didn't notice anything else! _Maybe_ it was a bit more sluggish that's about it. I'll see how bad it affected my fuel mileage after I gas up later today.


----------



## smj999smj (Jan 1, 2006)

Be interesting to see the exhaust emissions with a gas analizer before and after disconnecting the o2 sensor, especially if you could get the readings pre-cat.


----------



## jdg (Aug 27, 2009)

Plugged the O2 sensor back in, reset the light, drove it for another 50 miles (2x 25 miles to/from work), gassed up. Got about 28MPG which is just a hair below what I normally get and considering it's about 1F outside and I'm bucking snow and headwinds and got a bunch of winter weather gear in the trunk.
And ya, if I had the $$$, I'd like to see those emissions numbers too. They'd probably cause somebody from C.A.R.B. to have a seizure or something, but I'd be willing to bet that even without an O2 sensor, the numbers would be a heck of a lot better than say something from the '70's or the '80's. On that same note, I read somewhere that in the event of a failure of the front O2 sensor, some ECU's will revert to keeping tabs on the rear O2 sensor to see if they're in the ballpark as far as A/F ratios go. Obviously, not nearly as accurate, but something is better than nothing.


----------



## Jopet (Aug 10, 2009)

What is a good o2 sensor brand to buy. 

Need to get a new one. I replace my front April of last year and the o2 sensor died a few weeks back. I got the one named "Ultrapower". Is bosch a good brand for o2 sensors of sentras?

ty


----------



## Jopet (Aug 10, 2009)

one more thing, looking at the parts available online, I found most are described "California" or "Cali". Will this work for federal emissions?


----------



## smj999smj (Jan 1, 2006)

Federal and California emission certified vehicles usually run different O2 sensors. As far as brands, I prefer NTK (same company as NGK) or Denso and usually get pretty good prices at Rockauto.com. Some don't care for Bosch sensors but I never had any problem with them. Just go for direct fit type, not the universal sensors that have you use your original sensor's connector and have you splice all the wires. For the part number of NTK sensors for you car, use the part locator at NGK Spark Plugs USA.


----------



## Jopet (Aug 10, 2009)

thanks very much!!! BTW, the O2 sensor that died on me was from rockauto too. Just avoid that brand.


----------

