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P0422 code on an Audi A4 – investigation leads to turbo and cat replacement

January 4, 2010

Last year my wife’s daily driver, a 1999 Audi A4 Quattro with the 1.8L turbo charged engine began setting a check engine code P0422 which indicates not enough difference in value between the first and second oxygen sensor.   On OBD II cars, 1996 and newer, two oxygen sensors are used in the exhaust system in series mounted ahead and after the catalytic converter.  The first sensor provides data used to adjust the air / fuel ratio by the ECM while the second sensor’s data is compared to the first to ensure that the converter is effective in reducing the level of hydrocarbons.   If the values are too similar, the ECM sets a code indicating the converter may not be working.

As this code was intermittent in our case, I simply reset the code and planned to replace the oxygen sensors in the near future as a first step, and then the converter if needed as a second and more expensive step.   I will note that the car never ran as strongly as it should, and I suspected the turbo was not generating boost.  At 150K miles, I expected the turbo bearings were worn and needed to be replaced, and this might explain the lack of power and rich fuel condition leading to the P0422 code.   

The over rich condition also led to higher exhaust temps at the cat, as the downpipe eventually cracked around it’s circumference at the turbo flange.  I’m infering that the higher temps caused the failure after many heating / cooling cycles.   I initially planned to just remove the converter with intention of simply rewelding the flange.   While I had the converter out of the car I noted that it was stopped up – couldn’t see light through it.  I also found the round plunger from the wastegate actuator had come out of the turbo and been blown into the converter.  

Note the missing plunger / plug, the open port and the wastegate actuator with the hole (where the plug would mount) to the right side of the picture.  To the left side we can see the exhaust side turbine wheel.

Mystery of the non-performing turbo solved.  With the wastegate permanently open the exhaust was always in bypass and the turbo would not spool up – no boost.   These findings seemed to explain the lack of power, rich mixture, plugged up cat and the p0422 code.

I removed the turbo – a lengthy process, if fairly straightforward.   Instructions from a shop manual or All-Data would have been helpful, but I just grabbed the wrenches and jumped in.  The two things I would note are:

1) Drop the AC compressor out of the bottom of the car and suspend it with bungy cord.   Once this is out of the way, there is adequate room to work to remove a mounting brace to the turbo, the oil and coolant drain lines, and intake and pressure side hoses to the front of the turbo.  The pressure feeds for oil and coolant, along with the hoses to the wastegate controller can be removed from the top side.  Having a lift really helps as you can work more comfortably from a seated or standing position under the car.  It might be possible to do the work with ramps or jack stands while laying on your back, but I’d recommend against it.

2) There are coolant lines connected to the turbo charger.   A lot more coolant than one might expect will drain from these lines when you disconnect them.  Be prepared for it with a large drain bucket.  

While we could have welded the plunger back to the actuator arm on the wastegate, I noted the bushings on the bell crank assembly were worn as well.  These items could be addressed, but this seemed like a great time to just replace the turbo.  And, as long as I was replacing, why not upgrade from the factory K03 to a K04.  The K04 uses the same housing and bolts right up, but has larger internal components and a different A/R ratio.  

What a shiny clean, factory new turbo should look like – with all port covers and plugs in place to keep dust and debris out prior to installation.

This is what an intact wastegate assembly should look like.  Compare this to the failed K03 above.  

The factory converter is a pricey item, and I elected to go with a performance converter made from a generic high flow shell with pipe sections and flanges appropriately clocked and welded in place.  The assembly provided to be slightly longer than the factory configuration once installed, which meant that the factory pipe hanger at the transmission no longer bolted up and will need to be cut loose and rewelded.

I’ll share a follow up posting with hopefully favorable results after I finish refilling the oil, coolant and complete the road test.

9 Comments leave one →
  1. January 10, 2010 10:02 pm

    Its amazing that the engine could do much of anything. I can’t wait to see the outcome.

  2. January 11, 2010 2:18 pm


    Codes are gone. Drivability is waaay up. The car is now fairly responsive – not rocket compared to the EVO by any means, but still at lot more fun. I took it out and ran it hard on a test drive and when I came back, I popped the hood – the entire exhaust side of the turbo was glowing bright orange like a stove burner. I left the car idling and it quickly cooled. I understand why there are so many heat shields.(which I cursed and wrestled with during the install) I think I’ll way to get the shop wired and lifts in, but I’d like to upgrade the exhaust, change the intake side and maybe spring for a reflash / chip of the ECM to dial up the boost a bit.

    • January 14, 2010 11:32 am

      Sounds fantastic. It also sounds like you’ve been bit by the turbo bug!

  3. January 28, 2010 3:26 pm

    re-chip, throw on some hard pipes, and rev ‘er to the moon!

    i know what your garage needs… a dyno. 😉

  4. Severin permalink
    October 4, 2010 7:44 pm

    How much did this cost? I think my Audi has the same problem.. P0422 – except now my engine sounds like a motor boat..

    • October 5, 2010 6:27 pm

      Email me more details about your audi – I’ll see if I can help you out.

      Generally, look for the exhaust leak – at either end of the cat. If the gasket is blown out, that will throw off the O2 sensors and set the code. While I had mine apart, I found the converter brick was plugged up and that the wastegate was toast – I upgraded to the K04 turbo. Ebay is a good source for parts.

  5. July 8, 2014 11:45 am

    Thanks for your personal marvelous posting! I really enjoyed reading it,
    you’re a great author.I will make sure to bookmark your blog and will come back
    sometime soon. I want to encourage yourself to continue your great work, have a nice evening!

  6. February 27, 2015 10:30 pm

    Hey I was wondering if you were running a test pipe after you upgraded to the ko4. Did the p0422 code go away? Usually when I run a test pipe I use the spark plug defouler trick to trick the ecu but it didn’t work this time. I ended up replacing both 02’s but its still throwing a code.


  1. Audi A4 K04 Turbo upgrade / replacement « markitude

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