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Upstream vs downstream O2 sensors

Discussion in '1st Gen. Tacomas (1995-2004)' started by jaunty, Aug 27, 2016.

  1. Aug 27, 2016 at 8:41 AM
    #1
    jaunty

    jaunty [OP] Well-Known Member

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    Anyone know the difference between the upstream and downstream o2 sensors?

    My truck sounds like a box of rocks under load!

    I have the lean condition code p0171


    Thanks


    EDIT** will be going with Denso. The part changes after 2000 (more expensive)

    Denso 234-4161 Oxygen Sensor


    EDIT2**
    I found a small leak near my intake. A small crack in a hose. I also needed to clean the MAF better, there are some hidden wires inside, not just the bulb.
    Code is gone and so is box-of-rocks sound under load.
    HTH
     
    Last edited: Dec 29, 2016
  2. Aug 27, 2016 at 8:48 AM
    #2
    Itchyfeet

    Itchyfeet Well-Known Member

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    If you're looking at Bosch nope not even once.

    Denso or NTK
     
  3. Aug 27, 2016 at 8:52 AM
    #3
    jaunty

    jaunty [OP] Well-Known Member

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    But there is no way to tell from the code (p0171) which sensor up or down I'll need, correct?
     
    Last edited: Oct 18, 2016
  4. Aug 27, 2016 at 8:54 AM
    #4
    Skootter14

    Skootter14 Upon my signal, unleash Hell

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    have you tried cleaning your MAF sensor?

    I have a URD TCAI and the sensor gets dirty quicker than normal (especially during winter and salt/grime/slush is on the road). I get all sorts of different codes, all pertaining to lean, rich, banks issue, etc. I take the sensor out, spray it with cleaner, let air dry, reinstall and no issues. its become part of my normal maintenance.

    just be very careful and dont drop it or knock it since the bulb in there is sensitive and can break.

    a 4$ can of MAF cleaner is quick and cheaper than the O2 piece. I always try to start with the cheapest/quickest fix (as long as it could be a true fix) before I work my way up to bigger parts.

    just some advice. good luck
     
  5. Aug 27, 2016 at 8:56 AM
    #5
    jaunty

    jaunty [OP] Well-Known Member

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    Thanks I haven't done that in a few years. I'll give it a go.

    Basically my truck has been throwing that code when I get under load. Now it also sounds like a box of rocks under load. I think it's bad news. Im hoping the lean condition is what's causing that coffee grinder sound from down below!!!
     
  6. Aug 27, 2016 at 8:59 AM
    #6
    Nedrolls

    Nedrolls Well-Known Member

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    I believe the downstream is strictly for checking the cat and the upstream actually does the work?
     
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  7. Aug 27, 2016 at 9:12 AM
    #7
    cruiserguy

    cruiserguy Well-Known Member

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    The upstream Toyota calls 'air fuel sensor'. Threw me for a loop. They usually run about double the cost of the rear O2 sensor. Got my Denso one from Amazon for a hundred bucks. Took care of my MIL and my gas mileage is 30+% better. No joke.
     
  8. Aug 27, 2016 at 9:31 AM
    #8
    McMash

    McMash The only thing better than light bars? Sarcasm.

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    What he said. Their term for the front one is "Air/Fuel Sensor." And don't quote me, but I believe the P0420 code is the one for the rear/O2 sensor? At least, that's what mine threw. Haven't got the P0141. Yet.
     
  9. Aug 27, 2016 at 11:23 AM
    #9
    NightProwler

    NightProwler Well-Known Member

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    Coffee grinder, box of rocks? Kinda sounds like your cat(s) are done. As in the internals have broke loose and rattling around in there.

    And fyi, I had to go with a Bosch upstream as that was the only one that would fit. And a denso did fit the rear though. No issues with the Bosch yet. Not a lot of miles on it yet since though.
    And P0420 is for the rear. Could be cats, could be rear sensor, could even be Something else. But with that sound, I'm leaning towards cat. You might try cleaning the sensor like suggested but First time hearing that one. Otherwise you can t31st for backpressure to be sure. Or just fork out the $ for the rear sensor if you aren't mechanically inclined and start throwing money at it, thatd be the first thing to try. Try the cheapest options first. If all else fails (cleaning O2, maf, etc) get a back pressure test. If you don't wanna do it yourself, I imagine a shop shouldn't charge TOO much just for a back pressure test.. I thought my cats were clogged and ended up going with a whole new customs exhaust because I wanted one and had the excuse. But I believe it was just the rear sensor that was throwing the code. Because there was no increase in performance (as I thought I had a clogged cat and sluggishness to go with it) but no change at all with all new components. Oh well I've got all new stuff and a ba exhaust now. Heh.
     
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  10. Aug 27, 2016 at 11:35 AM
    #10
    cruiserguy

    cruiserguy Well-Known Member

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    I had the specific code for bank 1 sensor 1. That's why I bought Denso part number 234-9001 air/fuel sensor. I'm unfamiliar with other MIL codes being fixed by replacing this. I could see it affecting catalytic convertor things, I guess.
     
  11. Aug 27, 2016 at 11:52 AM
    #11
    jaunty

    jaunty [OP] Well-Known Member

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    I made a mistake... my code was 0171 for lean condition.

    I cleaned the MAF.. it was nasty.
    My exhaust manifold bolts (those 10mm ones) were a bit loose.

    Gotta do a test drive and hope it goes away, perhaps wishful thinking.

    As for the cat, this noise only happens when under strong load.. like going uphill. It doesn't occur when revving in the driveway, or with t-case in neutral and tranny in gear with a rev.
     
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  12. Aug 27, 2016 at 1:00 PM
    #12
    Skootter14

    Skootter14 Upon my signal, unleash Hell

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    if you had a "lean" condition, my bet is on the MAF sensor. That was the one code that tripped the most with my dirty sensor. cheapest and easiest fix. hope it works.
     
  13. Oct 7, 2016 at 11:16 AM
    #13
    jaunty

    jaunty [OP] Well-Known Member

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    I replaced the upstream sensor with the Denso part. So far so good! The code has stayed off for a week now. :fingerscrossed:
     
  14. Oct 7, 2016 at 1:03 PM
    #14
    keakar

    keakar Well-Known Member

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    I think I recall someone saying the front o2 sensor is the real sensor that does everything as far as the ECM stuff and the rear sensor is there to only read if there is and difference in the flow readings after the cat indicating cat issues
     
  15. Oct 7, 2016 at 2:14 PM
    #15
    Clay_916

    Clay_916 Well-Known Member

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    Rear sensor is only to monitor cat efficiency. It won't affect afrs. Front can be o2 or "afr" sensor, both of which are o2 sensors, narrow and wideband respectively. The front o2 sensor is how your engine reads and adjusts closed loop fuel trims. If you're having fuel issues in closed loop it is likely your front o2 sensor.
     
  16. Oct 8, 2016 at 8:28 AM
    #16
    Missbehaven

    Missbehaven New Member

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    Does anyone happen to have a picture of the location of the upstream o2 sensor? Is it underneath the front of the truck? Under the hood?
     
  17. Oct 8, 2016 at 8:44 AM
    #17
    Clay_916

    Clay_916 Well-Known Member

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    It'll be near the passenger front tire, just follow the tail pipe.
     
  18. Oct 8, 2016 at 8:47 AM
    #18
    Missbehaven

    Missbehaven New Member

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    Thank you! And the other one is located towards the rear?
     
  19. Oct 8, 2016 at 8:54 AM
    #19
    Clay_916

    Clay_916 Well-Known Member

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    They'll be on either side of the cat.
     
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  20. Oct 8, 2016 at 9:14 AM
    #20
    se7enine

    se7enine MCMLXXIX

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