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Thread: Engine Code Diagnosis

  1. #1
    Junior Member
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    Jan 2020
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    Engine Code Diagnosis


    After CEL came on on my VW Polo I took it to the garage to have it scanned.

    The fault codes I got were:

    P2096 - Post Catalyst Fuel Trim System Bank1 Too Lean
    P0420 - Catalyst System Bank1 Efficiency Below Threshold (Short Circuit to B+)

    Any Ideas?

    2010 Polo 1.2, Petrol (59PS)

  2. #2
    Administrator Crasher's Avatar
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    Apr 2006
    Nottingham, England
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    That could be an air leak upstream or a faulty pre cat lambda sensor that is not yet bad enough to trigger an error. I have seen a LOT of these with the exhaust flexi section cracked which allows air in and confuses the ECU with the post cat sensor Oxygen values.
    "I disapprove of what you say, but I will defend to the death your right to say it"

    Evelyn Beatrice Hall (28 September 1868 13 April 1956)

  3. #3
    Administrator Keithuk's Avatar
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    Jun 2012
    Stoke on Trent
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    VAG Error Code: 18528
    EOBD II Error Code: P2096

    Fault Location:
    Post Catalytic Converter Fuel Trim, Bank 1 - too Lean

    Possible Cause:
    Malfunction Indicator Lamp (MIL)(K83) active.

    Low fuel pressure.
    Misfire condition.
    Large vacuum leak.
    Air leak near sensor.
    Lean air/fuel ratio.
    Faulty Front Oxygen (Lambda) Sensor.
    Catalytic converter exhaust leak.

    The catalyst's efficiency is demonstrated by its ability to oxidize CO and hydrocarbon emissions. The Powertrain Control Module (PCM) compares the output signals of the front and rear oxygen sensors to determine whether the output of the rear sensor is beginning to match the output of the front oxygen sensor. Air/fuel mixture compensation keeps the frequency of the front oxygen sensor high due to the changes from rich-to-lean combustion. The catalyst causes the rear oxygen sensor to have a lower frequency. As the catalyst wears, the rear oxygen sensor’s signal trace begins to match the front oxygen sensor’s signal trace. That is because the catalyst becomes saturated with oxygen and cannot use the oxygen to convert hydrocarbon and CO into H2O and CO2 with the same efficiency as when it was new. A completely worn catalyst shows a 100% match between the frequency of the front and rear sensors.

    VAG Error Code: 16804/001056
    EOBD II Error Code: P0420

    Fault Location:
    Catalyst System, Bank 1 - Efficiency Below Threshold/Lower Limit Exceeded/Upper Limit Exceeded

    Possible Cause:
    Malfunction Indicator Lamp (MIL)(K83) active.

    Leakage in Intake and/or Exhaust System.
    Catalyst faulty.
    Oxygen Sensor(s) faulty.
    Oxygen Sensor(s) Control faulty.
    Exhaust tube.
    Intake air leaks.
    Fuel injectors may be faulty.
    Fuel injector leaks.
    Spark plugs may be faulty.
    Improper ignition timing.

    Possible Solutions:
    Check Intake and Exhaust System for Leaks.
    Check Catalyst.
    Check Oxygen Sensor(s).
    Check Oxygen Sensor(s) Control.
    Perform Oxygen Sensor(s) Aging Check.

    Special Notes:
    For 1.8T's experiencing this fault code, this thread may be useful: Aftermarket DP/Cat -DTC - Cat efficiency below threshold - Who has this DTC and with DP/Cat
    Check the Official Factory Repair Information for any Technical Product Information (TPI)/Technical Service Bulletins (TSBs) that apply to dealer level Engine Controller flash updates for the vehicle in question
    VW Vehicles with the Most TSBs .

    Tech Notes:
    As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the Oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

    When is the code detected?:
    Three way catalytic converter Bank 1 does not operate properly.
    Three way catalyst converter Bank 1 does not have enough oxygen storage capacity.

    The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensor (HO2S) 1 (front O2 sensor) and heated oxygen sensor 2 (rear O2 sensor).
    A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of Oxygen (Lambda) Sensor 2 (G130). As oxygen storage capacity decreases, the Oxygen (Lambda) Sensor 2 (G130) switching frequency will increase.
    When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.

    As taken from my new EOBD II Fault Codes Software
    2010 Golf GTD (170) CBBB, 5 Door, Shadow Blue (P6/D5Q), Transmission Code: KXZ, Flat Tyre Indicator, Basic Protection Pack + VCDS.

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