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P0420 code

Discussion in '1st Gen. Tacomas (1995-2004)' started by trmarshall1, Apr 13, 2012.

  1. Apr 13, 2012 at 8:44 AM

    trmarshall1 [OP] The Least Interesting Man In The World

    Apr 7, 2012
    First Name:
    San Jose, CA
    02 Tacoma 4x4 TRD Supercharged
    4" Pro-Comp Lift, TRD cat back exhaust, grey wire mod, clutch pedal linear spring mod
    I had a friend run a check on my Taco with his code reader and the code was P0420, now that in my opinion would be either an upstream or downstream O2 sensor malfunction. Would it be advisable to replace both sensors at the same time, or determine which one is faulty and just replace that one? Thanks for the help people, I appreciate it.
  2. Apr 13, 2012 at 8:48 AM

    Casey05Taco Well-Known Member

    Aug 26, 2008
    First Name:
    Penngrove, CA
    05 Tacoma Prerunner
    trd seat covers, bed mat, tinted windows, debadged, mud flaps removed, tinted tails, blacked out center console, Black Rims, 6000k HID in highs and fogs, blacked out front emblem, Pioneer AVH-4300DVD, Gauge Cluster, lower valence, retrofitted, audio system, Maxtrac Spindles, Billies
    i do believe just yesterday another 1st gen asked this question. Let me search and see if i can find that thread and maybe the answer will be there

    edit: found this.
    Problem fixed!!
    Finally the problem was only an exhaust leak. The flange gasket between the second Cat and the extension, right before the second O2 sensor was finish. I replaced it, erase the codes and voila!! done... the check engine light is off since 2 month now...

    The reason the check engine light goes on with P0420 code is because when you are driving at a constant speed, the flow in exhaust creates a vacuum and pull fresh air from exterior by the hole and falls the o2 sensors reading...

    Verify leaks by blocking muffler and check for very small leaks, specially near o2 sensors!!"
  3. Apr 13, 2012 at 8:52 AM

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

    May 8, 2008
    First Name:
    Tenoe, AZ
    2013 Rubicon Unlimited,
    4.10 gears, sliders, and lots of buttons.
    A code P0420 may mean that one or more of the following has happened:
    • Leaded fuel was used where unleaded was called for
    • An oxygen sensor is not reading (functioning) properly
    • The engine coolant temperature sensor is not working properly
    • Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
    • Retarded spark timing
    • The oxygen sensors in front and behind the converter are reporting too similar of readings

    Some suggested steps for troubleshooting a P0420 error code include:
    • Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
    • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
    • Inspect the downstream heated oxygen sensor (HO2), replace if necessary
    • Replace the catalytic converter
  4. Apr 13, 2012 at 8:53 AM

    Vassily28 Well-Known Member

    Nov 24, 2011
    Quebec, Quebec, Canada
    11 TRD SPORT 4x4 Quadcab LB
    TRD supercharger with 2.85" URD stealth pulley and OEM FPR mod. URD fuel pump upgrade, URD TB plate adapter 70mm, FJ/4 runner 70 mm Throttle body, URD exhaust cam gears URD performance thermostat TRD CAI 1st gen. with custom cover (coupe), ProDry S + Filter skin, Doug thorley long tube headers, URD spec U exhaust Icon LT 2.5 front coilover, Timbren front bumpstops, Total Choas UCA, Deaver custom leafpack 3 inch. lift, Icon 2.5 VS series rear resishocks, Timbren rear bumpstops, ARB front bumper with WARN 8500 lbs winch,Hella 500 blackmagic, AVID rear bumper, Allpro front IFS skid + Trans. skid + Transfercase skid. Total chaos LCA skid, SS braided brake line, URD O2 sims, URD MAF controller, Wetokole front seatcovers, Innovate MTX-L, MTX-D AFR and boost gage.
    P0420 Catalyst System Efficiency Below Threshold Bank 1

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