PO422 - Bad Sensor?
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  1. #1
    2nd Gear
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    Nov 2004
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    PO422 - Bad Sensor?

    I've been running a LaBree High Flow Cat on my 1998 1.8t (APR, ITG filter, TT DV, silicon vac hose) for 11 months w/ no CELs. Last weekend a CEL appeared and the Auto Zone guy pulled a PO422. I was told that this code indicates that the Cat could be defective...another explanation might be that an oxygen sensor has failed and is reporting the Cat as below threshold when the Cat is OK. Given that my Cat has less than 12+K miles on it and the motor runs well I suspect it is a sensor. Is there anything else I should check (the Autozone ODBII book had other reasons for this code - large vacuum leak, misfire, engine oil in exhaust guides)?

    Since the sensors for an AEB are not cheap ($110+) I'd rather not swap both of them out if I only need one. Are both the pre-Cat and post-Cat sensors the same? If so I could buy a one new sensor and swap it in until the CEL disappears, right?

    All help is appreciated.

    Loren

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  3. #2
    2nd Gear
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    Follow up

    VAG COM of the car showed:

    16806 - Main Catalyst: Bank 1: Efficiency Below Threshold
    P0422 - 35-00 - -

    At first glance this would indicate that my catalytic converter was bad. But the efficiency (performance condition) of the cat is determined in the ECU by comparing the readings of the O2 sensors positioned before and after the cat...so a failing sensor that is reading inaccurately can make a perfectly functional cat look like it is malfunctioning. Daryl at Tantrumworks (the shop that sold me the cat, very helpful) felt that the cat was not at fault and suggested that replacing the rear O2 sensor would solve the code.

    Using VAG COM tests, both O2 sensors continued to pass and the cat continued to fail - so I was not able to conclusively identify either sensor as bad. But since the car had 135K, the O2 sensors appeared to be original (with an average working lifespan of about 100K) and the cat had only 12K, I decided to replace both sensors and see what happened. Picked up Bosch OEM replacements on sale at Autohaus AZ (front #15160 $100.49 shipped, rear #13842 $115.24). Swapped them out and the cat passed subsequent VAG COM tests. Woo-hoo!

    The front sensor is used by the ecu to read if fuel mixture is lean or rich and to make needed adjustments (rear is used only for the cat efficiency measure) - I've read that an old, failing 02 sensor can cause fuel economy to dip. My mileage had fallen noticeably recently - I'm hoping it was due to the old sensor and now improves.

    Hope this helps someone.
    Last edited by Salaryman; 07-14-2006 at 11:17 PM.

  4. #3
    Super (Charged) Moderator, Grinding Gears...gone fishing! ScottPassat2.8's Avatar
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    i think your #'s will most likely go up.
    i had a bad one on my a4 2.8 and what a mess that was.

  5. #4
    2nd Gear
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    Nov 2004
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    Bonus!

    Got the car smogged today and it passsed with flying colors, as expected. The biggest surprise was how much peppier the engine response was - it was very noticeable. Even TIP tranny shifts at various speeds were smoother and more seamless (at WOT the car had a tendency to hesitate as it upshifted or downshifted).

    Gas mileage just driving around town (very unscientific - using memory and the on-dash MPG indicator) appears to have gained 2-3 MPG - a 10% gain would be awesome!

    Based on this experience my advice would be -

    a) If you own an AEB at close to or over 100K replace the O2 sensors, even if they are not creating a CEL. The engine performance and fuel economy gains make it worth the $100 per sensor.

    b)If you own a model that uses a $25 sensor I'd make replacing it a regular part of your regular maintenance schedule - maybe every 60K...maybe sooner!

  6. #5
    Neutral
    Join Date
    Aug 2007
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    6
    hello i had the same thing the po422 but according to what ive read vw have alot of problems with the sensors. im trying to change my sensors before i actually do something with the cat can someone explain to me how to change my o2 sensor the code that popped was po422 main cat. efficiancy low bank1 thank u

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