keep getting po420 need a little help
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This is a discussion on keep getting po420 need a little help within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; I've had my car for little over a month & engine light just came on I used my cobb & ...

  1. #1
    Registered User wrx rookie's Avatar
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    keep getting po420 need a little help

    I've had my car for little over a month & engine light just came on I used my cobb & it came up with po420 when i cleared it it stayed off for about a week but then came back the car has some mods could my cat on the down pipe be bad already? or
    could this be an o2 sensor? also looking to change my rear diff & tras lube any suggestions? Is it ok to run royal purple in both with out adding any of the shock proof in the trans?
    Any help is much appreciated Love this car & love the site



    here's a list of my mods if it will help

    Blitz Intake
    Port and Polished Headers
    Cobb Up-pipe
    Cobb Downpipe with High Flow Cat
    Unorthodox Racing Underdrive Pulleys
    Cobb Accessport v2
    Perrin Turbo Inlet Pipe
    Full JIC Magic Titanium Exhaust

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  3. #2
    Master Baiter EJ257's Avatar
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    My guess would be the rear O2 sensor being fouled. If your HFC from the Cobb DP is bad, it would cause a fouling of the sensor. Replace the sensor and see if that helps.
    2005 WRX STi (Mods | Virtual Dyno)

    Resident Tuner @ WTF Tuning

    "Never trust anything that bleeds for a week and lives ..."

    UNYSOC

  4. #3
    Moderator Donkey's Avatar
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    It helps if you list the year and type (WRX/STI) vehicle.I'm assuming you would be running the stage 2 map with those vehicle mods.If not then that would be your problem. If you are running the stage 2 map then make sure the rear O2 sensor is plugged in.If so contact Cobb. The stage 2 map is supposed to "fix" that DTC. That DTC usually happens when you install a TBE. I'm assuming you read the instructions fully and the map notes as well

    CEL Codes Defeated [WHEN USING AS BASE MAP] (** means new to latest revision):
    P0037 - Rear O2 Sensor Heater Control Circuit Low
    P0038 - Rear 02 Sensor Heater Control Circuit High
    P0137 - Rear O2 Sensor Circuit Low Voltage
    P0138 - Rear O2 Sensor Circuit High Voltage
    P0139 - Rear O2 Sensor Circuit Slow Response
    P0420 - Catalyst System Efficiency Below Threshold
    P0545 - Exhaust Gas Temperature Sensor Circuit Low
    P0546 - Exhaust Gas Temperature Sensor Circuit High
    P1301 - Misfire Detected (High Temperature Exhaust Gas)
    P1312 - Exhaust Gas Temperature Sensor Malfunction
    P1544 - Exhaust Gas Temperature Too High


    "stock intake, stock fuel injectors, and a turbo-back exhaust system. MUST USE FACTORY OR COBB SF INTAKE SYSTEM ONLY, NO OTHER AFTERMARKET INTAKES ARE CERTIFIED COMPATIBLE WITH THIS CALIBRATION."
    Resident Adviser @ WTF Tuning, LLC
    2002 FORESTER S
    1998 FORESTER S




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