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Purpose of 2nd O2 sensor & Pre-Cat

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    Purpose of 2nd O2 sensor & Pre-Cat

    OK people I'm at a loss right now.

    What exactly is the point of having the 2nd O2 sensor and the Pre-cat?

    I ask this cause I had my pre-Cat removed and a downpipe w/ an O2 sensor bung put in its place. Right after i had the downpipe installed I noticed that I had the following codes:
    P0135 - (Heated Oxygen Sensor Heater circuit fault - Bank No. 1) and
    P0134 - (O2 sensor circuit no activity detected Bank 1).
    I simply thought this was because the O2 sensor would be reading around the same thing. But... I had a leak in me exhaust right at the flange of the downpipe (away from the exaust manifold).

    After I took my car back to the shop and had them seal up the leak, I reset the comp and the check engine light has been off for almost 300 miles. Thats great and all but WTF. I'm not understanding why there isn't a code.

    I'll be driving to NJ from MD so I'll see if it comes back on.
    ED
    '05 6s 3.0L MT & '98 Pro DX 1.5L AT

    Mazda Enthusiast Club Member
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    #2
    I'm guessing the Pre-Cat is to reduce emissions but why does the 1.5 need it and the 1.8 doesn't.

    The O2 sensor is kinda checking for the efficiency of the Pre-Cat, so as someone stated on this forum removing it should make the O2 sensor think it's less efficient. If that's the case I should get a check engine light immediately.
    ED
    '05 6s 3.0L MT & '98 Pro DX 1.5L AT

    Mazda Enthusiast Club Member
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    2nd Gen Protege Parts Catalog project in progress
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    Comment


      #3
      so i guess people are at a loss with this question or
      just don't care

      Well ... guess i'll do some more research
      ED
      '05 6s 3.0L MT & '98 Pro DX 1.5L AT

      Mazda Enthusiast Club Member
      Click for my CarDomain page


      2nd Gen Protege Parts Catalog project in progress
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        #4
        Many manufacturers use 2 02 sensors, and like you just have guessed, it is for emissions. Your ECU would get the readings from both sensors, and with pre-cat, there would be two different readings. According to those reading your ECU would make the necessary changes to keep the emissions at the right levels. There are other things going on too, but I am not too experienced in this area.
        "Probable impossibilities are to be preferred to improbable possibilities."

        Comment


          #5
          I passed NJ emissions with just the main cat. And it's not the standard cat it's Magnaflow's Hi-Flow cat.
          ED
          '05 6s 3.0L MT & '98 Pro DX 1.5L AT

          Mazda Enthusiast Club Member
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          2nd Gen Protege Parts Catalog project in progress
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            #6
            dumb o2 sensors

            indeed. the second oxygen sensor is important. not only does it regulate emissions, but it also helps control air/fuel ratio. i wish there was a way to bypass the pre cat and the 2nd o2 sensor with out sending active codes to the computer.(make check engine light come on).
            I live my life a quarter pounder at a time. And for those 500 calories or more, I'm free. I need FRIES! Two of them. The big ones. Oh, and I need them tonight. You're lucky the double shot of BBQ sauce didn't blow the seam on your nugget box. There she is, 2 pounds of pure beef. My dad ate it in 9.0 seconds flat. Check it out, it's like this. If I lose, winner takes my happy meal. But if I win, I take the burger and the toy. To some people, that's more important.

            ._________________________
            |.....Overnight....................| ||
            |.....Japan Parts.................| |'|";,___.
            |_..._...____________======||_|_|...,]
            "(@)'(@)""'''''''''''''"'''"**|(@)(@)*****"(@)
            Oh and by the way that shot in your banner with Vin Diesel's car getting shot, thats a civic not an altezza.

            Comment


              #7
              Originally posted by wheresthericego
              i wish there was a way to bypass the pre cat and the 2nd o2 sensor with out sending active codes to the computer.(make check engine light come on).
              I don't have a pre-cat anymore and I'm not getting any check engine light. It's been over 700 miles now.

              It might have been the shop I took it to. They made me get all this work done to replace the 1st O2 sensor, carbon clean engine, ignition wires and oilchange just to get rid of my check engine light, which after research was my pre-cat going bad.
              I gave them problems when they told me I had to pay for them to replace the 2nd O2 sensor, so they said they'd do it for free. It looked like some of the wiring was surrounded be electrical tape but I didn't have time to checkit out. They might have gotten me a universal O2 sensor with a chip to make the computer happy. Would've been cheaper for them.
              ED
              '05 6s 3.0L MT & '98 Pro DX 1.5L AT

              Mazda Enthusiast Club Member
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              2nd Gen Protege Parts Catalog project in progress
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              Comment


                #8
                I checked out my car not too long ago and the shop I took it to did replace my second O2 sensor with the cheap generic kind and just spliced the wires.

                I'm down to only one error code P0140 - (O2 sensor CKT no activity Bank 1 Sensor 2), if I remember correctly.

                Here's a summary of stuff I've done:
                1. Initially had catylyst under threshold error because my pre-cat was messing up
                2. I said to hell with the pre-cat, took it out and replaced it with a downpipe
                3. Same catylyst under threshold error came back after weeks of driving
                4. Used Apex mil eliminator, now P0140 error code appears

                Is it possible the crappy O2 sensor the shop installed doesn't read the same as the extremely expensive ones meant for our car, so that when I use the MIL eliminator chip it gives the totally wrong signal?

                I think I have my old 2nd O2 sensor lying around.

                So here's what I'll do:
                1. Remove MIL chip and see if old code returns.
                2. Remove crappy O2 sensor and replace with my old O2 sensor check to see if code is consistent with 1.
                3. Try MIL chip with old O2 sensor
                4. If all that fails, ask what type of reading the O2 sensor is supposed to give under normal operating conditions.
                Last edited by loganss; 12-14-2004, 08:19 AM.
                ED
                '05 6s 3.0L MT & '98 Pro DX 1.5L AT

                Mazda Enthusiast Club Member
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                2nd Gen Protege Parts Catalog project in progress
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                Comment


                  #9
                  loganss,

                  so are you getting a CEL light? i mean if you are not why are you still experimenting? just curious as to where you got your downpipe? were they pre-made or you had a shop fabricate it?

                  1996 1.5L Mazda Protege DX MTX
                  Mods:
                  DX converted to an LX (F & R seats and All Power), 2" Exhaust from the Catback, EuroReverse Glow Gauges, JDM Clear Side Markers, 17" MB Motoring Rims, K&N air filter 33-2134, All Clear Lenses, Front Strut Bar, MS Sideskirts, JDM Taillights, JDM Turbo Diesel Hood Scoop, OEM Door Visors, and Ducktail.
                  http://www.cardomain.com/id/beetle_orange

                  Comment


                    #10
                    Originally posted by beetle_orange
                    loganss,

                    so are you getting a CEL light? i mean if you are not why are you still experimenting? just curious as to where you got your downpipe? were they pre-made or you had a shop fabricate it?
                    Getting error code P0140.
                    Downpipe was custom made by a local shop. I searched for a pre-made one but with no luck. I think I have picks of the downpipe in my car domain page
                    Last edited by loganss; 12-14-2004, 08:19 AM.
                    ED
                    '05 6s 3.0L MT & '98 Pro DX 1.5L AT

                    Mazda Enthusiast Club Member
                    Click for my CarDomain page


                    2nd Gen Protege Parts Catalog project in progress
                    WTB: Gray 97/98 Protege Cupholder and Door Trim

                    Comment


                      #11
                      saw your cardomain and saw the downpipe. does your car have a rear sway bar? i noticed mine didnt have one but saw bruce's thread (converting rear drum's to disc's) and his pro had a bar wherein the ends were connected / attached to the strut housing.

                      1996 1.5L Mazda Protege DX MTX
                      Mods:
                      DX converted to an LX (F & R seats and All Power), 2" Exhaust from the Catback, EuroReverse Glow Gauges, JDM Clear Side Markers, 17" MB Motoring Rims, K&N air filter 33-2134, All Clear Lenses, Front Strut Bar, MS Sideskirts, JDM Taillights, JDM Turbo Diesel Hood Scoop, OEM Door Visors, and Ducktail.
                      http://www.cardomain.com/id/beetle_orange

                      Comment


                        #12
                        The DX doesn't have the strut end links (think that's the name) to connect a bar to. I think bruce has the end links for a bar because he replaces his stock struts for the Mazdaspeed adjustable struts. Best bet would be to contact Bruce.
                        ED
                        '05 6s 3.0L MT & '98 Pro DX 1.5L AT

                        Mazda Enthusiast Club Member
                        Click for my CarDomain page


                        2nd Gen Protege Parts Catalog project in progress
                        WTB: Gray 97/98 Protege Cupholder and Door Trim

                        Comment


                          #13
                          Originally posted by TheMAN
                          What a load of bull**** from people who have no idea how this **** works.... why doesn't anyone research the **** first before trying to be helpful?
                          You know what, Edwin, fine. Next time I will make sure that a complete research gets done before any one answers any questions here. Unless you of course you decide to beat us all to it with your extensive knowledge. You do know that people come here for help, sometimes. We might as well have them sent back to the search engines to research thier sh*t, or just forward all the questions to you.
                          "Probable impossibilities are to be preferred to improbable possibilities."

                          Comment


                            #14
                            Ok so no that everyone has the arguing out of there system I have some questions/announcments.

                            Post #8 and 10 were changed. I checked the error code and it is slightly different from what I previously posted. I have error code P0140 O2 sensor CKT no Activity (Bank 1 Sensor 2) not P0134.

                            Question: If the computer determines that the difference between the two O2 sensors isn't enough, what can I do to make it so the computer thinks the two reading are the correct difference.

                            And please lets keep the arguing to a minimum. We're all trying to help each other out here.
                            ED
                            '05 6s 3.0L MT & '98 Pro DX 1.5L AT

                            Mazda Enthusiast Club Member
                            Click for my CarDomain page


                            2nd Gen Protege Parts Catalog project in progress
                            WTB: Gray 97/98 Protege Cupholder and Door Trim

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