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  1. #1
    Veteran Member Four Rings getslideways's Avatar
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    Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

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    So i was reading this DIY for how to install a Diode in the rear O2 sensor wiring so that the ECU sees the voltage it wants, assumes the cat is fine, and then stops giving "efficiency below threshold" codes:

    http://www.audiworld.com/tech/eng53.shtml

    and then stated reading this post about how to check the O2 sensor pins to see if your sensor is working, and what each pin does:

    http://www.audiworld.com/tech/eng101.shtml


    so what i am wondering is, does anyone know if it would be possible to simply clip the sensor harness leaving only a little bit of wire behind the connector, then connect those wires together in such a way that with the right diode/resistor combination inbetween them the ECU sees the voltages/current it wants, and assumes a correctly working, properly functioning o2 sensor is present and detects a working cat, when in fact it is not?

    basically physically removing the whole rear o2 sensor completely? Plugging in a "fake" o2 sensor, and then just putting a plug in the 02 sensor exhaust bung?

    anyone have any insight, ideas, or knowledge on the situation? if it would work, or why it would/does not work.

    in theory it would be great, no fault codes, no codes caused by its absence, and one less wire bundle cluttering up the engine bay
    Last edited by getslideways; 10-26-2009 at 12:32 PM.

  2. #2
    Senior Member Three Rings
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    I don't have an answer for you, but I'm wondering the same thing.

    I saw your Lambo link and... wow.
    Parting out 1999.5 a4 1.8tqm pm me for parts and any reasonable offer is accepted!

  3. #3
    Veteran Member Four Rings 99blueb5's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    or why dont you do the easy way and get two spark plug anti foulers and run those, its like 6bucks and it about as easy as it gets?
    99 A4 1.8TQM:
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  4. #4
    Veteran Member Four Rings getslideways's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    just for something different, and because the antifoulers don't always work for everyone so i was hoping for an alternative and rather than guessing and test fitting various combination of antifoulers it would be great if you could just tell the car what it wants to hear and be done with it

    i have also seen a lot of antifouler mods that will trigger the cel if driving at one rpm for a long time or that will rip the bungs out from the added weight and leverage hanging off the side as a result of heat cycles weakening it

    so again not saying antifoulers are bad, or saying they dont work, just looking for an alternative

    in my particular case i think my rear o2 sensor is bad anyway, so i would rather try to make a "fake" one if possible and try something different, than spend the cash for a brand new sensor that i am just trying to trick anyway and then the antifoulers when all i really want is to make a light turn off lol

  5. #5
    Veteran Member Four Rings Militant-Grunt's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    A tuner can code them out of the ECU. I don't have numerous sensors connected on my car, they're all coded out. I don't believe you can put a resistor on a 02 sensor mainly because you have variable data coming in from the sensor, while for connectors that turn things on or off you can.
    -MilitantGrunt- Audi USA. Certified Audi Technician
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  6. #6
    Veteran Member Four Rings getslideways's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    the writeup in the first link made it sound like the ecu just wanted to see a certain value (example: "anything under .45 volts means there is enough oxygen present that the cat must be doing its job")

    but i see what you are saying, that instead maybe the ecu is looking for a signal out of the post cat sensor that is some lowered factor of the precat signal, right?

    so instead of this like (how i thought the first article was explaining it):
    Precat Voltage___PostCat Voltage
    ___1.2______________.45
    ___1.8______________.45
    ___2.2______________.45
    ___2.8______________.45
    ___4.9______________.45

    The ECU wants to see something like this where there is a consistent difference between the two (in this example post cat signal is .5 volts less than the pre cat signal):

    Precat Voltage___PostCat Voltage
    ___1.0______________.50
    ___1.5______________1.0
    ___2.0______________1.5
    ___2.5______________2.0
    ___4.0______________3.5


    or maybe the second value has to be some percentage of the first value, like this (in this example postcat signal is 50% of the precat signal):

    Precat Voltage___PostCat Voltage
    ___1.0______________.50
    ___1.5______________.75
    ___2.0______________1.0
    ___3.0______________1.5
    ___4.2______________2.1

    Does anyone know what it is?

    I wondering if you could just take the precat sensor signal, and splice it and drop that voltage by some constant or factor and use the new dropped voltage line as the "post o2 sensor" output, and use a diode to make sure that new voltage value only goes one way (to the ecu and then keeps from messing up the precat sensor signal)?

  7. #7
    Veteran Member Four Rings mholme's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    I've never seen anyone successfully do the resistor mod, including myself. You're better off tuning it out or using the non fouler trick.
    08 S4- Stock

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  8. #8
    Veteran Member Three Rings SpeedWorksAuto's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    holy crap @ lambo link
    2001 A4 1.8T - 2.7TT Swap w/ custom Single Turbo setup

    http://www.audizine.com/forum/showth...-build)/page16

  9. #9
    Registered Member Two Rings
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    whats the lambo link????

  10. #10
    Veteran Member Four Rings ThatA4T's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    look at his sig
    Double DIN swap, pioneer avh4000, TONS of suede, 15% tint, Forge 007, apr/vdo, rs4 grille, GIAC chip, Depo ecodes with 55w 5000k HID's, ST Coilovers, 034 street density motor and trans mounts, 034 snub w/ bracket, 034 aluminum subframe bushings, valeo single mass cluth/flywheel setup, new control arms and axles, evo ms CAI, powderoated snazzy stuff, euro trunk, multi colors, Shaved exhaust cuttout with hidden turn down tips (in progress... again)


    its only just begun...

  11. #11
    Senior Member Three Rings
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    No each pop of each cylinder is seen as a wave of exhaust flowing through the pipe, the ecu is looking for varying voltage after each burst with both o2 sensors. Cutting the o2 wire won't work you would get a no activity code, a range/performance code and a heater circuit code. Spicing into the pre cat o2 sensor would be a bad idea because you are going to change the load changing the voltage the ecu sees causing incorrect fuel trims.

    The reason non foulers work is because the head of the o2 sensor is pulled out of the direct exhaust stream but it still is picking up whiffs causing the ecu to see activity.
    Driver Mod....

    --- (.)(.) ---
    ---- ).( ----
    --- ( Y ) ---

  12. #12
    Veteran Member Three Rings B5 SLPR's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    I know APR has a tune that shuts off the second 02 sensor for a TP
    "I like to party"
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  13. #13
    Veteran Member Four Rings getslideways's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    is there a way to copy the precat signal without changing or altering it?

    there are a few places that will sell specific resistors that mimic the o2 heating sensor to address the ecu's heating circuit request

    I also found this other guy who claimed he was able to build one, but the site he bought his parts from now claims the EPA made them stop selling all of their o2 simulator components

    http://forums.audiworld.com/showthread.php?t=1574487

  14. #14
    Veteran Member Four Rings getslideways's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    does anyone happen to have a log where both those voltages were measures? i would be interesting to see the difference between the two

  15. #15
    Senior Member Three Rings
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    There is a company that makes 02 simulators that are specifically designed for Audi/Vw and Porsche.
    Driver Mod....

    --- (.)(.) ---
    ---- ).( ----
    --- ( Y ) ---

  16. #16
    Senior Member Three Rings
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    the thing about APR is they don't charge for the program, but they do charge for labor. I got a quote for $70 from an APR dealer in new hampshire. I'm sure the price varies depending on dealer, but damn- 70 bucks?
    Parting out 1999.5 a4 1.8tqm pm me for parts and any reasonable offer is accepted!

  17. #17
    Veteran Member Four Rings getslideways's Avatar
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    Re: Can You Delete the Rear O2 Sensor Completely With Resistors and Diodes?

    Quote Originally Posted by Toyotatech View Post
    There is a company that makes 02 simulators that are specifically designed for Audi/Vw and Porsche.
    do you have a link? i only saw one but they said they discontinued them


    Quote Originally Posted by myb5quattro View Post
    the thing about APR is they don't charge for the program, but they do charge for labor. I got a quote for $70 from an APR dealer in new hampshire. I'm sure the price varies depending on dealer, but damn- 70 bucks?
    i take it that is if you already have a previously installed APR chip right? or will they program that code into a stock ecu?



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