P0420 Code on B7

[betyouaint]

So, shortly after fixing my start-up and running issue by replacing the MAF sensor, I was hit with another CEL for Catalyst Efficiency below Threshold (P0420). This was around 80 miles after the MAF replacement, which was purchased used from a breakers yard. I cleared the code and got maybe another 100 miles out of the car before it reappeared again. This cycle has continued for the last couple of weeks. Unfortunately, inspection is due next month so I need to get this resolved with some urgency.

I've found lots of instances of P0420 on the B6 but not so many on the B7. Lots of possible causes are discussed and often with other symptoms too. I haven't identified any other symptoms although once I thought the engine stuttered slightly before the CEL came on.

I ran the lambda sensor checks in VCDS and they come up good. I also logged air/fuel/lambda parameters on a couple of journeys where the CEL was triggered but nothing jumps out as being unusual around the time of the CEL being triggered. That said, I'm not sure what to look for and there are periods when one of the post-cat sensor values (resistance..?) is completely empty. However, this happens throughout the logs and it doesn't necessarily coincide with the CEL.

Obviously, I'm suspicious of the new (used) MAF but I'm not sure that this would/could cause this code?

Also, the flex pipe on my exhaust is shot and has been for around 2 years. So far this hasn't caused any problems and maybe this is the first signs of deterioration. Can downstream exhaust leaks trigger P0420?

I should probably mention that the car in approaching 150k and is on original cat and O2 sensors but I don't wish to just change them out as a primary means of diagnosis.

[jamerican1]

As the resident Jamaican, I think your car likes weed. Sorry I have nothing helpful to add.

[betyouaint]

I can't make much of the VCDS readings. I don't see much fluctuation from the O2 sensor readings as might be expected from conventional systems. That said, I may be looking at the wrong voltages. I'm not even sure I have a reading for the upsteam/pre-cat (Sensor 1) output. There is one value that's called "Bank 1 Voltage of Oxygen Sensors" (G033) and also a voltage reading specific to Sensor 2, but nothing that states Bank 1 Sensor 1 voltage. The sensor 2 voltage actually does drop from around 0.7V to around 0.1V or less on occasion which I believe indicates the presence of oxygen and a possible bad cat. By the final sample this value is constantly

There is one value (G041 Resistance Bank 1 Sensor 2 kOhm) that often disappears. It doesn't even show zero but is completely blank (see second sample below) Does anyone know what this might indicate? There's also another value (G037 Bank 1 Sensor Value) that constantly reads 0.008


In the meantime, I've rolled the dice on a possible fix using the O2 extender...

https://www.europaparts.com/034motor...-111-z002.html


Below are a number of samples from recent logs containing lambda related value data

[betyouaint]

Ummm... I did final some correlation between the G033 and G036 as shown in the sample below. A high G033 almost always results in a low G036. If only I knew what G033 actually represented...? Is this bank 1 sensor 1 or a combination of both sensors (as suggested by the name).

[betyouaint]

The cheap option isn't always the easy option. Fitting the spacer doesn't look like an easy job. I can just about get a 22mm wrench on it but there's no room to turn it. I have a shorter adjustable wrench but I'm not sure it will take the strain even if I could get enough leverage to break things loose. Is there a secret technique or do I need to pull the cat?

Just found this...

http://www.audizine.com/forum/showth...-Cat-O2-Sensor

Hopefully, that will give me a little extra wiggle room.

[betyouaint]

Nope... I can just about get my fat forearms in and I adapted a wrench to fit but no go. Then I dropped a torx bit into the undershield and had to pull that off. Figured I'd do an oil change while I had it on the ramps with the shield off.

I'm now weighing my options. If I'm gonna pull the cat, I'm thinking it might be worth just throwing in a test pipe and the spacer. I've had a new downpipe sitting in the garage for almost a year waiting for me to install it.

Does the spacer eliminate the cat efficiency code with a test pipe? Or will I need to get the post cat O2 coded out with a tune?

[betyouaint]

Anyone used CATACLEAN before? My mechanic friend told me it was worth a try; only $20. Just waiting to run my tank down to a 1/4 as per instructions before I throw it in. Seen some positive outcomes reported from a number of online sources and some not-so-positive outcomes from people expecting a miracle cure-all. Guess I'll report back when I've run it through.

[80sGuy]

Get rid of that used MAF. Faulty parts can cause a daisy-chain of troubles for these cars.

[Okedokey]

Anyone used CATACLEAN before? My mechanic friend told me it was worth a try; only $20. Just waiting to run my tank down to a 1/4 as per instructions before I throw it in. Seen some positive outcomes reported from a number of online sources and some not-so-positive outcomes from people expecting a miracle cure-all. Guess I'll report back when I've run it through.

How well did it work?

[betyouaint]

No luck with the Cataclean. The snake oil seems not to work or at least not in this case. Saw lots of good reports of the miracle cure but not for me.

I've since fitted the O2 sensor spacer and that seems to have done the trick.