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Thread: What's the purpose of the Center O2 sensor?

  1. #1
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    What's the purpose of the Center O2 sensor?

    On the 2005 VW Jetta 2.5L I recently purchased ... it uses 3 Oxygen sensors, one in the exhaust manifold ... one in the CAT itself ... and one located after the CAT. Most cars, including my 2007 VW Passat, only have two O2 sensors.

    What purpose does the center O2 sensor serve, located on the CAT?

  2. #2
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    Hi Geoffzie,
    I am not familiar with US emission cars but suspect you are looking at the Exhaust Gas Temperature sensor.
    Regards HMC

  3. #3
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    The part looks like a normal O2 sensor ... So an O2 sensor can measure exhaust temperature? Would the center O2 sensor be involved in a P0420 (or P0430) Fault Code then?

    I'm just trying to understand how the system works with a center O2 sensor. I know on most cars, the sensor in the exhaust manifold, nearest the engine, measures fuel / air ratio, and sends that signal to the ECU so that it might adjust the fuel mixture accordingly.

    The rear O2 sensor measures the oxygen content AFTER the CAT to verify that the CAT is working. If it senses the same gas content AFTER the CAT ... as BEFORE the CAT ... it sets a P0420 or P0430 indicating the CAT is not working. At least that's my limited understanding.

    So how does the Center O2 sensor play into that ... if at all?

  4. #4
    NostraJackAss Jack@European_Parts's Avatar
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    On the 2005 VW Jetta 2.5L I recently purchased ... it uses 3 Oxygen sensors, one in the exhaust manifold ... one in the CAT itself ... and one located after the CAT. Most cars, including my 2007 VW Passat, only have two O2 sensors.

    What purpose does the center O2 sensor serve, located on the CAT?

    The part looks like a normal O2 sensor ... So an O2 sensor can measure exhaust temperature? Would the center O2 sensor be involved in a P0420 (or P0430) Fault Code then?

    I'm just trying to understand how the system works with a center O2 sensor. I know on most cars, the sensor in the exhaust manifold, nearest the engine, measures fuel / air ratio, and sends that signal to the ECU so that it might adjust the fuel mixture accordingly.

    The rear O2 sensor measures the oxygen content AFTER the CAT to verify that the CAT is working. If it senses the same gas content AFTER the CAT ... as BEFORE the CAT ... it sets a P0420 or P0430 indicating the CAT is not working. At least that's my limited understanding.

    So how does the Center O2 sensor play into that ... if at all?

    It is used as a method of monitoring the pre catalyst and secondary cat AECD's, in addition defines an SULEV and can be observed in VCDS 0x01-08-045>048 depending on banks. In this case just bank one.
    Also could be a amplitude cross check across the bank to qualify a monitor or OEM part from what I've observed.
    It is effectively a lower GHG vehicle, which carries a longer warranty for emissions useful life, under CARB/EPA/NHTSA for conformity unilaterally & while supposedly running cleaner as per the CAA usually with zero EVAP as a PZEV in comparison to ULEV or LEV.

    A Google search of the acronyms should help more definement.
    The RTFB will also have further explanation, as do the SSP's.
    Answered from my smartass phone yay!
    NostraJackAss Has Spoken!
    Last edited by Jack@European_Parts; 12-07-2017 at 12:30 PM.
    European Parts Emporium/Performance / Immobilizer Solutions LLC
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  5. #5
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    Quote Originally Posted by HMC View Post
    Hi Geoffzie,
    I am not familiar with US emission cars but suspect you are looking at the Exhaust Gas Temperature sensor.
    Regards HMC
    I seriously doubt this relatively low-stressed, naturally aspirated engine has an actual EGT sensor.

    -Uwe-
    Ceterum censeo, delenda est Daesh.

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  7. #6
    NostraJackAss Jack@European_Parts's Avatar
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    Oh and FYI the rear OXS is the most superior for mixture control, not the front & because the CAT's clean up the reading adding clarity, in addition how EVAP is determined in specification.
    European Parts Emporium/Performance / Immobilizer Solutions LLC
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  8. #7
    NostraJackAss Jack@European_Parts's Avatar
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    People may find this helpful...........

    https://pdfs.semanticscholar.org/938...088cc74762.pdf



    Model Validation
    Validation of the three-way catalytic converter model is performed by comparing measured
    vehicle dynamometer emissions with simulated emissions during the cold transient
    phase of an FTP75 certification driving cycle. The vehicle chosen for this purpose
    is a Volkswagen Jetta equipped with the low-end torque Volkswagen 2.0L-2V-85 kW
    engine (165 Nm at 2600 rpm). A thorough revamp of the engine, its control, the gasoline
    supply and the exhaust system made this vehicle the first Partial Zero Emission
    Vehicle (PZEV) of Volkswagen [178]. To ensure ultra-fast catalyst light-off the engine
    is started in rich mode (
    l = 0.6 – 0.75) with secondary air being added into the
    manifold targeting for exhaust compositions of
    l = 1.05 – 1.25.
    air pump
    oxygen sensor (HEGO)
    oxygen sensor (HEGO)
    2nd catalytic brick
    1st catalytic brick
    temperature sensor (Pt200)
    oxygen sensor (UEGO)
    secondary air injection
    Figure 7.1: Schematic sketch of Bosch Motronic ME7.1.1 system architecture (source:
    Robert Bosch GmbH, GS/ESK) - incomplete.
    92
    UEGO (mounted to the exhaust manifold) HEGO (downstream 2nd brick)
    3” x 4.66”, 900 cpsi, 2.5 mil 4.5” x 4.66”, 900 cpsi, 2.5 mil
    200 g/ft
    3, Pd:Rh = 14:1 100 g/ft3, Pd:Rh = 14:1
    HEGO (mid brick)
    TC2
    temperature sensor
    TC1
    TC3 TC4
    1st brick 2nd brick
    Figure 7.2: Volkswagen’s Jetta 2.0l-2V-85 kW SULEV compliant exhaust system
    equipped with thermocouples (TC1 - TC4).
    The air-gap insulated manifold acts during this phase as a so-called thermoreactor,
    completing the combustion process in a hot flame [178]. The temperature sensor located
    downstream in the takedown pipe checks for proper operation of thermoreactor
    and secondary air pump (figure 7.1). Since the SULEV emission thresholds (figure 1.1)
    have to be fulfilled for mileages of 150 thousand miles without further facilitation, the
    catalytic converter system is located in the underfloor position to avoid severe aging
    conditions under full load operation. The system contains two bricks of the same
    diameter. The first brick is shorter and has twice the precious metal loading of the
    second brick (figure 7.2). There is one oxygen sensor with step characteristic mounted
    between the two bricks and another one behind the second brick.
    Modal measurements of the raw gas entering the catalytic converter system and of the
    gas behind the first brick are performed at 1 Hz by means of Fourier-Transformation-
    Infrared-Spectrometry (FTIR) with Volkswagen’s System for Emission Sampling And
    Measurement (SESAM) [8].
    Accumulated mass of carbon monoxide, hydrocarbons and nitrogen oxides behind the
    first brick during the cold transient phase (
    t = 0. . .507 s) of the FTP75 test procedure
    are shown in figure 7.3. During cold start – which only needs a couple of seconds
    for this application – a major fraction of total carbon monoxide and hydrocarbons is
    emitted. After light-off, the temperatures are high enough for complete combustion
    under lean conditions. Hydrocarbons are even converted under rich conditions: only
    a slight and almost constant increase in hydrocarbon mass is registered after the first
    brick – emphasizing the importance of converter light-off on hydrocarbon emissions.
    The emissions of carbon monoxide and nitrogen monoxide after the first brick are substantial.
    Especially nitrogen oxide emissions are significant between
    t = 180. . .300 s
    and thus due to warmed-up catalyst performance. In general, NO
    x control strategies
    focus on closed-loop, stabilized engine operation [174].
    93
    0 60 120 180 240 300 360 420 480
    t [s]
    1
    2
    3
    4
    m
    CO, mNOx
    [g]
    0.00
    0.01
    0.02
    0.03
    0.04
    0.05
    0.06
    0.07
    0.08
    m
    C3Hx
    [g]
    20
    40
    60
    80
    100
    v [km/h]
    C
    3Hx
    CO
    NO
    x
    Figure 7.3: Measured accumulated carbon monoxide, nitrogen oxide and hydrocarbon
    mass behind first brick during the cold transient phase of the FTP75 driving cycle.
    The stoichiometry of the catalyst inlet emissions are crucial for three-way catalytic
    emission control and thus for a sound description of the catalyst behavior modeled
    in this work (see chapter 4). It is evident from figures 2.2 and 2.3 that the dynamic
    response of the oxygen concentration signal is significantly slower than the dynamic
    response of the polar species measured by FTIR technique. The dynamics of all analyzed
    emission components are damped compared to the relative air to fuel ratio signal
    provided by the linear oxygen sensor. Thus the oxygen concentration in the feed
    is calculated from equation 2.17 based on the linear oxygen sensor (UEGO) signal
    which exhibits a much better dynamic response to changes in gas phase composition
    (figure 7.4)
    1. The hydrogen concentration is assumed to be one third of the carbon
    monoxide concentration (see chapter 3).
    With the calculated oxygen concentrations the resulting set of engine out emissions still
    underestimates the dynamics in engine out hydrogen, carbon monoxide, hydrocarbon
    and nitrogen oxide emissions but obeys the fast dynamics of the measured relative air
    to fuel ratio.
    1
    The relative air to fuel ratio signal is obtained from an extra Bosch LSU sensor not connected to the
    engine control unit. This sensor is powered up well in advance to cranking the engine – thus providing
    sound readings from the very first second. The obvious offset of the measured oxygen concentration
    (not used in this work) is probably caused by a leakage in the vehicle roll exhaust sample piping.
    94
    0 60 120 180 240 300 360 420 480
    t [s]
    0.0
    1.0
    2.0
    3.0
    4.0
    5.0
    y
    O2
    [%]
    calculated
    measured
    20
    40
    60
    80
    100
    v [km/h]
    Figure 7.4: Converter inlet oxygen concentrations calculated from equation 2.17 based
    on the linear oxygen sensor (UEGO) signal compared with the respective oxygen concentration
    measured by means of a paramagnetic oxygen analyzer.
    7.1 Thermal Behavior
    Feed temperature estimates
    Even though temperature readings of thermocouples
    placed within the tube in front of the first brick are available (thermocouple TC2 in
    figure 7.2), simulations based on these values result in way too low catalyst bed temperatures:
    due to radiative heat losses temperature sensors positioned within a hot gas
    phase surrounded by colder walls do have lower body temperatures (and thus provide
    lower readings) compared to the gas phase they are positioned in [112, 121]. Thus
    a feed gas temperature estimate believed to be more realistic is obtained by ”interpolation”
    of the erroneous reading in front of the catalyst (TC2) and a hotter – also
    erroneous – reading even further upstream (TC1 in figure 7.2). The empirical formula
    T
    g,in 0.3TTC1+0.7TTC2 yields reasonable feed temperature values for the first brick.
    This can be seen in the following, where start-up measurements of the FTP75 cycle of
    figure 2.2 are compared to simulations (figure 7.5).
    Heat of reaction for oxygen storage and release
    In order to check the influence of
    the heat of reaction of oxygen storage and release, several simulation results are compared
    with temperatures measured at two positions within the first brick. A quite good
    description of the temperature progression with time at different positions within the
    catalyst is obtained with the model described in chapter 4 and the parameters obtained
    95
    in chapter 5 and 6
    2. The heat of oxygen storage is varied between a value of -760
    (equivalent to the heat of reaction 3.14), a value of -200 kJ/(mol O
    2) and zero.
    25
    50
    75
    100
    v [km/h]
    200
    400
    600
    800
    T [
    oC]
    D
    hR,os = -760 kJ/(mol O2)
    200
    400
    600
    800
    T [
    oC]
    D
    hR,os = -200 kJ/(mol O2)
    0 60 120 180 240 300 360 420 480
    t [s]
    200
    400
    600
    800
    T [
    oC]
    D
    hR,os = 0 kJ/(mol O2)
    x = 15mm (measured)
    x = 15mm (simulated)
    x = 60mm (measured)
    x = 60mm (simulated)
    Figure 7.5: Measured and simulated temperatures within the first brick, assuming different
    values of the enthalpy
    DhR,os of oxygen storage reaction.
    The value of -200 (close to the value of -220 kJ/(mol O
    2) associated with palladium
    oxidation) has been reported for calorimetric properties of ceria and ceria-zirconia
    based palladium three-way catalysts [180]. The literature value of -200 kJ/(mol O
    2)
    provides the best description of the thermal behavior under warmed-up conditions.
    2
    Since the precious metal loading in the first brick is twice the loading of the catalyst the kinetic
    parameters are obtained for, all kinetic rate constants are assumed to be twice as high.
    96
    With both a higher and a lower heat of oxygen storage and release significant deviations
    between measured and simulated temperatures arise – especially during deceleration
    associated with fuel cut and subsequent rich purges (figure 7.5, 120 s
    < t < 360 s).
    7.2 Cold Start Conversion
    Due to the thermoreactor cold start strategy the first brick experiences temperatures
    in excess of 700
    C within a couple of seconds after cranking the engine (figure 7.5).
    The critical cold start phase is passed within less than 20 s just before the first road
    load is put on the engine. Figure 7.6 allows a comparison of measured and simulated
    combustible species concentrations during this phase.
    0 5 10 15
    t [s]
    C
    3H8
    CO
    0 5 10 15 20
    t [s]
    100
    200
    300
    400
    500
    y
    C3Hx
    [ppm]
    C
    3H6
    engine out measured
    mid brick measured
    mid brick simulated
    0.0
    0.5
    1.0
    1.5
    2.0
    2.5
    y
    H2
    , y
    CO [%]
    H
    2
    Figure 7.6: Measured and simulated hydrogen, carbon monoxide and hydrocarbon
    emissions after the first brick (”mid brick”) during cold start.
    Hydrocarbons are mainly emitted during the very first seconds of the driving cycle and
    to a lesser degree during the corresponding seconds of the hot transient phase. More
    than 10 different hydrocarbon species are measured by SESAM. They are grouped into
    a fraction with relatively low catalytic ignition temperature represented by propene
    and another fraction with relatively high catalytic ignition temperature represented by
    propane.
    97
    During light-off propene shows a similar behavior as carbon monoxide with full conversion
    after approximately 8 s. Propane light-off is somewhat delayed with full conversion
    after 10 s. The evident time shift between simulated and measured peak hydrocarbon
    emissions is due to the catalyst’s ability to store hydrocarbons in an intermediate
    temperature range – a phenomenon not covered by the mathematical model. As can
    be seen, hydrocarbon storage has no substantial influence on total hydrocarbon emissions
    as long as hydrocarbon desorption occurs well in advance of catalyst light-off.
    7.3 Transient Phase Conversion
    During light-off the rates of catalytic combustion are limited by the low temperature.
    In the hot catalyst under rich feed conditions after removal of residual oxygen, combustion
    rates are limited by the rate of oxygen release from the oxygen storage. Under
    lean feed conditions the conversion of nitrogen oxide to nitrogen is limited by the rate
    of oxygen uptake.
    0 60 120 180 240 300 360 420 480
    t [s]
    0.950
    0.975
    1.000
    1.025
    1.050
    lambda [-]
    post 1st brick (simulation)
    25
    50
    75
    100
    v [km/h]
    0.900
    0.925
    0.950
    0.975
    1.000
    1.025
    1.050
    lambda [-]
    engine out (measurement)
    Figure 7.7: Relative Air to Fuel ratio across the first brick.
    If oxygen uptake and release were high enough all deviations from stoichiometric composition
    would be buffered, resulting in a steady
    l = 1 signal behind the catalyst. The
    simulated exhaust composition after the first brick shows this behavior during idling
    98
    with low load after the first cruising period (around
    t 150 s). Higher load operation
    is accompanied with lean and rich breakthroughs (figure 7.7).
    Significant nitrogen oxide breakthrough occurs during the following high speed operation
    between second 200 and 300 (figure 7.3). It is evident from the time resolved
    traces in figure 7.8 that these emissions are due to isolated peaks. While overall conversion
    is well in excess of 80%, oxygen storage during lean engine operation seems
    to be not sufficiently high to buffer all the oxygen – hindering complete nitrogen oxide
    reduction.
    During acceleration with high engine load the first brick emits significant amounts of
    carbon monoxide under rich conditions. The accumulated mass of carbon monoxide
    shows a pronounced increase around
    t 350 s (figure 7.3). These few seconds add
    approximately 20% to the mass accumulated during the cold transient phase. An explanation
    will be given by considering the (simulated) change of stored oxygen.
    It can be seen from the measured pre-first brick gas composition and the calculated
    post-first brick gas composition (figure 7.7) that the average engine exhaust is slightly
    rich. The exhaust composition is controlled such that the oxygen sensor’s voltage
    behind the second brick is always 600-650 mV to ensure further high conversion of
    nitrogen monoxide in the second brick [178].
    With the catalyst storage being initially completely oxidized due to lean cold start with
    secondary air injection, the slightly rich operation afterwards results in an average
    decrease of the stored oxygen (figure 7.10). During the second hill (
    t 150. . .300 s) an
    average oxidation extend of roughly 35% is maintained providing high oxygen uptake
    rates at still good oxygen release rates. During deceleration with fuel cut (
    t 320 s) the
    storage material is rapidly oxidized. During the following rich purge (
    t 320. . .350 s)
    oxygen is released from the storage until the storage is virtually empty. At
    t = 350 s
    the engine accelerates the vehicle with rich combustion and relatively high load. The
    carbon monoxide conversion now suffers from a completely reduced storage material,
    unable to compensate oxygen shortage in the feed. This explains the strong carbon
    monoxide increase at about 350 s (figures 7.3 and 7.9).
    After coldstart the measured hydrocarbon conversion is almost complete (figures 7.3
    and 7.11). Using the kinetic data for steam reforming and water gas shift reaction determined
    separately in section 5.4.2 and given in table 5.1, the simulated propane conversion
    would have been too low. In other words: measured hydrocarbon conversion
    under transient rich conditions with the Volkswagen Jetta 2.0l SULEV development
    application is much better than the one predicted by simulation based on kinetic data
    retrieved from steady state experiments. It is evident from data discussed in chapter 8
    and appendix C that these differences most likely arise from sulfur poisoning: Kinetic
    measurements discussed in chapter 5 are performed with an exhaust sulfur content of
    y
    SO2 = 14 ppm, while the certification driving cycle measurements discussed here are
    performed with a much lower sulfur level. Furthermore in the presence of sulfur the
    catalyst transforms into a less active state during prolonged steady state rich operation.
    During real engine operation the catalyst is subjected to rich conditions no longer than
    40 sec (see figure 7.7). Sulfur blocking the catalyst’s active sites as e.g. hydrogen
    99
    500
    1000
    1500
    y
    NO [ppm]
    post 1st brick (measurement)
    25
    50
    75
    100
    v [km/h]
    500
    1000
    1500
    2000
    2500
    3000
    3500
    y
    NO
    +
    [ppm]
    engine out (measurement)
    180 240 300 360 420 480
    t [s]
    500
    1000
    1500
    y
    NO [ppm]
    post 1st brick (simulation)
    Figure 7.8: Comparison of measured nitrogen monoxide emissions in front and behind
    the first brick with simulated emissions behind the first brick.
    100
    0.5
    1.0
    y
    CO [ppm]
    25
    50
    75
    100
    v [km/h]
    0.5
    1.0
    1.5
    2.0
    y
    CO
    +
    [ppm]
    180 240 300 360 420 480
    t [s]
    0.5
    1.0
    y
    CO [ppm]
    engine out (measurement)
    post 1st brick (measurement)
    post 1st brick (simulation)
    Figure 7.9: Comparison of measured carbon monoxide emissions in front and behind
    the first brick with simulated emissions behind the first brick.
    101
    0 60 120 180 240 300 360 420 480
    t [s]
    0.00
    0.25
    0.50
    0.75
    1.00
    X
    OSC [-]
    Figure 7.10: Simulated averaged oxidation extent of the storage material.
    sulfide are swept away in the subsequent lean phase. Therefore propane conversion
    was calculated with the same rate parameters as propene in figure 7.11. Nevertheless
    some break-through of hydrocarbons is simulated during the rich acceleration at about
    350 s.
    7.4 Discussion
    With model parameters and properties obtained from the suppliers (e.g. substrate cell
    density and wall thickness, washcoat loading, precious metal loading), from literature
    (e.g. heat capacities, transfer coefficients, oxygen diffusivity within oxygen storage)
    and kinetic and capacity data from isothermal measurements performed on a sample
    with half the precious metal loading and a 25 % lower specific geometric surface area
    the behavior of a full size catalytic converter operated under highly transient thermal
    and stoichiometric conditions can be described reasonably well. This is in particular
    remarkable since one single set of species conversion kinetic parameters obtained
    from both fuel lean (oxygen saturated catalyst) and fuel rich (oxygen depleted catalyst)
    conditions (table 5.1) is applied. During Volkswagen Jetta 2.0l SULEV FTP
    drive cycle simulation however the catalyst does change between periods of oxygen
    release and oxygen uptake. Hence the oxygen storage is usually not completely filled
    or completely empty but somewhere inbetween (0
    x 1, see figure 7.10).
    While the independently parameterized model provides a good description of the cold
    start behavior and the thermal behavior throughout the cold transient FTP cycle quantitative
    differences between measurement and simulation are evident from a comparison
    of transient carbon monoxide and nitrogen oxide emissions on a peak to peak basis
    (figure 7.9 and 7.8). With the significantly higher sulfur content applied throughout
    all kinetic measurements, the scale up by more than two orders of magnitude, poor
    time resolved emission analysis, lack of hydrogen concentration analysis and a questionable
    flow distribution within the first brick (see inlet cone geometry figure 7.2) the
    degree of carbon monoxide and nitrogen oxide conversion prediction is believed to be
    remarkably good. Drive cycle simulations with similar models reported in literature
    provide roughly the same accuracy [126, 146, 4].
    102
    500
    1000
    y
    C3Hx
    [ppm]
    25
    50
    75
    100
    v [km/h]
    500
    1000
    1500
    2000
    2500
    3000
    y
    C3Hx
    +
    [ppm]
    180 240 300 360 420 480
    t [s]
    500
    1000
    y
    C3Hx
    [ppm]
    engine out (measurement)
    post 1st brick (measurement)
    post 1st brick (simulation)
    Figure 7.11: Comparison of measured hydrocarbon emissions in front and behind the
    first brick with simulated emissions behind the first brick. After coldstart engine out
    hydrocarbons are assumed to be propene only.
    103
    The simulations underestimate the warmed-up propane conversion under rich conditions
    compared to the measured – almost complete – alkane conversion. Similar shortcomings
    in predicting transient improved hydrocarbon conversion under rich conditions
    are reported in literature [166]. Steady state experiments with a rich bias to mean
    air to fuel ratio (e.g.
    l =0.98 0.01, f =1 Hz) carried out in this work (section 5.4.2)
    maintain the catalyst in an oxygen depleted state over its full length. With the Volkswagen
    Jetta 2.0l SULEV application described before the oxygen storage is controlled at
    an intermediate oxidation state during transient operation by applying low frequency
    oxidation and reduction cycles with reasonable amplitude. It has been reported that the
    overall conversion of a three-way catalytic converter system can be enhanced by prolonged
    rich/lean phases as long as full system break through is avoided (e.g. [90, 111]).
    Further experiments towards the understanding of this transient improved rich conversion
    phenomenon are discussed in chapter 8 and appendix C.

    European Parts Emporium/Performance / Immobilizer Solutions LLC
    Certified Master Trained ASE/SAE/NASTF Legal Factory Authorized/Licensed GeKo/FaZit # 403738
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    www.FixMyEuro.com <<<<<CLICK HERE! or vwemporium@aol.com ( JPPSG & Unverified members need not PM me & Please don't email for free tech support...use the forum )
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  10. #8
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    Here's the reason I asked.

    On several cars (both VW and Toyota) that had P0420 or P0430 fault codes, I've spliced a Radio Shack p/n 276-1141 3-Amp 50V Barrel Diode into the return signal line of the rear O2 sensor going back to the ECU. It fools the ECU into thinking the CAT is working good ... because the ECU is looking for a reduced voltage return signal. Hence, no check engine light. This assumes that your problem isn't a clogged CAT, and that the CAT is still in good physical condition with no external damage.

    I did this on my "Passetta" and it works fine ... and eliminates the P0420 Fault Code. And it's MUCH cheaper than a new CAT!

    Here's the write-up I found on an Audi site that explains the process ...

    Code:
    The rear oxygen sensor is a 4-wire heated unit. There are two Black, a White & Grey wire. They are:
    
    Grey: reference signal from ECU
    Black: output signal going to ECU
    2 White wires: 12v heater element power & ground
    
    I slid back the rubber boot and cut the BLACK wire. This is the O2 sensor (+) output. GREY is reference voltage, and both WHITES are the heater circuit. 
    Cut the BLACK wire, crimp in the diode with the silver bar facing the ECU side (towards the connector).
    So ... I asked the question concerning the 2005 Jetta 2.5 with a P0420 Code, since it has the center O2 Sensor (none of the other cars I've done this to had a center O2 sensor). I simply wanted to know what relevance the center O2 sensor had on the "CAT - Checking" process.

    Although, after looking at it closer today, I may have to just replace the CAT ... it does seem to be "rattling" too (172,000 miles). So my question may be academic.

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