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    Installed Rear (Downstream) O2 Sensor

    I received a MIL while driving the other day. I had it read and the code came up: P0421 - Warm-up catalyst system efficiency below threshold. According to the FSM, this indicates a faulty rear (downstream) O2 sensor. This is the first O2 failure I've had in the 15-year life of the car - not bad.

    I purchased a new (4-wire) sensor for $98.22 including state theft. This O2 sensor screws into the front pipe which connects the front catalytic convertor (what Mazda calls the Warm-up 3-way catalytic convertor) to the rear (downstream) catalytic convertor. I put the car up on jack stands and went to work. I really hate working under the car this way.

    Now, removing either O2 sensor from an aged exhaust system can sometimes be problematic (rust, corrosion, etc.) but in this case the old device came out relatively easily and in one piece. Why? Because about 5-years ago I replaced my rear (downstream) catalytic convertor and at the same time had to replace the front pipe which means I had to remove and reinstall the O2 sensor which probably loosened things up.


    I installed the new device and now everything is back to normal. No more MIL.

    Talking to some techs I was advised that if one O2 sensor malfunctions, the next one is not far behind so soon I will replace the front (upstream) device, as well, which screws into the exhaust manifold. The front sensor is much more expensive which is why I'm holding off for awhile. However, a failed front sensor is more disruptive than the rear since it has more of an effect on maintaining the proper air/fuel mixture.

    All in all a fairly straightforward procedure since I didn't have any removal problems.

    Happy Motoring!
    02 DX Millenium Red - The Penultimate Driving Machine
    MP3 Strut Tower Bar kit; Cusco Front Lower Arm Tie Bar
    MSP Springs, Struts, Stabilizer Bars, Trailing Links, #3 Engine Mount
    Kartboy Stabilizer Bar Bushings; Nyloil Shifter Bushings; Red Line MT-90 Gear Oil
    MP3 Shifter, Knob and Aluminum Pedal Set
    Suvlights HD Wiring Harness; Osram Night Breaker H4 Bulbs; Exide Edge AGM Battery
    Summer: 5Zigen FN01R-C 16 x 7" Wheels; Yoko S.drive 205/45-16s
    Winter: Enkei OR52 16 x 7" Wheels; Falken Ziex ZE-912 205/45-16s
    Modified OEM Air Intake; Racing Beat Exhaust System; Techna-Fit SS Clutch Line
    Denso SKJ16CR-L11 Extended Tip Spark Plugs; Magnecor Wires
    Power Slot Front Brake Rotors; Techna-Fit SS Brake Lines; Hawk HPS Pads
    Red Line Synthetic Engine Oil; C/S Aluminum Oil Cap
    Cyberdyne Digital Gauges: Tach; Ambient Air Temp; Voltmeter

    #2
    did you by any means inspect the front cat to ensure its still in good shape? they tend to melt/break down on the front pass side of the unit.
    ---Has ClubProtege helped you in someway? show your support by Contributing--- Click Here---

    1992- project FE3..... 313 WHP @ 9.3psi




    I pet my dash when I get into the car..."good car"
    he actually has a mazda tree, parts grow on it

    Comment


      #3
      Yes, I did inspect the front cat and it looked to still be in good shape as far as I could tell. Thanks for your suggestion I'll keep my eye on it.

      Happy Motoring!
      02 DX Millenium Red - The Penultimate Driving Machine
      MP3 Strut Tower Bar kit; Cusco Front Lower Arm Tie Bar
      MSP Springs, Struts, Stabilizer Bars, Trailing Links, #3 Engine Mount
      Kartboy Stabilizer Bar Bushings; Nyloil Shifter Bushings; Red Line MT-90 Gear Oil
      MP3 Shifter, Knob and Aluminum Pedal Set
      Suvlights HD Wiring Harness; Osram Night Breaker H4 Bulbs; Exide Edge AGM Battery
      Summer: 5Zigen FN01R-C 16 x 7" Wheels; Yoko S.drive 205/45-16s
      Winter: Enkei OR52 16 x 7" Wheels; Falken Ziex ZE-912 205/45-16s
      Modified OEM Air Intake; Racing Beat Exhaust System; Techna-Fit SS Clutch Line
      Denso SKJ16CR-L11 Extended Tip Spark Plugs; Magnecor Wires
      Power Slot Front Brake Rotors; Techna-Fit SS Brake Lines; Hawk HPS Pads
      Red Line Synthetic Engine Oil; C/S Aluminum Oil Cap
      Cyberdyne Digital Gauges: Tach; Ambient Air Temp; Voltmeter

      Comment


        #4
        awesome!!
        ---Has ClubProtege helped you in someway? show your support by Contributing--- Click Here---

        1992- project FE3..... 313 WHP @ 9.3psi




        I pet my dash when I get into the car..."good car"
        he actually has a mazda tree, parts grow on it

        Comment

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