P0420 Catalyst Code

[daughtersA4]

I recently performed the waste gate rod modifications on my 2012 A4 and that all went great. I got a P0420 code shortly after performing the modification. Not sure if it’s coincidence or if it’s come as a result of something I did while it was apart. I’ve never had this code before. Car runs fine but would prefer the CEL be off. Ive read online it’s most
Likely the cat but wondering if anyone had this code before and if the o2 sensor could be causing it. I don’t see a lot of issues related to this code online. Any help is appreciated.

[Stazi]

Did you disconnect the rear O2 sensor and either didn’t reconnect it or maybe damage the wiring to it when you did the fix?


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[daughtersA4]

Did you disconnect the rear O2 sensor and either didn’t reconnect it or maybe damage the wiring to it when you did the fix?


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I was really careful. I didn’t remove the after cat o2 when I did the job. I also cut the ties that secure the cable slack to give me movement to do the job. There are no visible signs of damage. One thing to note is that the frequency of the CEL coming on is sporadic after I clear it. Sometimes it takes 4-5 days to come back on and sometimes it comes back on right away. Not sure if that means anything.

When I had the cat disconnected the core of the cat appeared to be in good shape.

[old guy]

P0420 is almost always from a defective catalytic converter. A damaged or disconnected wire gives a different code (open circuit, plus to ground etc). The ECM runs regular checks on the cat. If it fails two tests in a row you will get a CEL. If it passes two consecutive tests the CEL will go away. Consequently you will often see the CEL come and go with increasing frequency until it simply stays on all the time.

Did you remove the turbo or exhaust to adjust the wastegate? A small leak in the exhaust can also cause the P0420 as the ECM reads the errant O2 level being caused by the leak.

[daughtersA4]

P0420 is almost always from a defective catalytic converter. A damaged or disconnected wire gives a different code (open circuit, plus to ground etc). The ECM runs regular checks on the cat. If it fails two tests in a row you will get a CEL. If it passes two consecutive tests the CEL will go away. Consequently you will often see the CEL come and go with increasing frequency until it simply stays on all the time.

Did you remove the turbo or exhaust to adjust the wastegate? A small leak in the exhaust can also cause the P0420 as the ECM reads the errant O2 level being caused by the leak.

Thanks for this. I removed the cat at the turbo only when I did the waste gate fix. I used a new gasket on the turbo/cat flange on reassembly. I don’t hear a leak and I double checked the nuts again after reading on another forum it could be an exhaust leak. Is there any way to verify an exhaust leak other than hearing it?

[old guy]

Several years ago I suspected a small exhaust leak on my A4 B6. I was able to confirm it by having my son sit behind the car at idle and use the soles of his shoes to temporarily block the exhaust flow. I was listening under the hood and was able to hear a slight hissing sound coming from the turbo to cat gasket.

Replaced the gasket and all was good.

[daughtersA4]

Several years ago I suspected a small exhaust leak on my A4 B6. I was able to confirm it by having my son sit behind the car at idle and use the soles of his shoes to temporarily block the exhaust flow. I was listening under the hood and was able to hear a slight hissing sound coming from the turbo to cat gasket.

Replaced the gasket and all was good.

I'll give your suggestion a try.

Something just crossed my mind I want to run it by everyone. The turbo to cat gasket I received from the dealer when I recently did the waste gate mod looked a little odd to me. It fit but it didn't look like the original one I took off. The original gasket I took off was a single steel gasket. The new gasket appeared to be made up of two pieces laid on top of each other but partially attached to each other. I could pull them apart in certain parts around the radius of the gasket but it was connected in other parts. It appeared to be two gaskets that there were semi attached to each other. Does this sound right to anyone? I didn't really question it until now. Could I have gotten a bad gasket? Did I install two gaskets by accident?

Edit - I just looked up the gasket online and it appears to be correct, made up of two pieces. A European design no doubt....

[Stazi]

Those are the factory style with the double layer steel. No issue there.


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[daughtersA4]

Those are the factory style with the double layer steel. No issue there.


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Yep thanks. Just looked it up online and confirmed that.

[BSieben]

Wastegate mod? Tell me more? I think my leak is at the wastegate. Rod has a lot of play. Will this beautiful-all-too-good-to-be-true "wastegate mod" fix this play issue?