Car: 2000 A4 quattro 1.8t
Mileage: ~51,000

CEL came on today while cruising on the highway. Used my bluetooth code reader to discover a P0422 Main Catalyst Efficiency Below Threshold (Bank 1) code.

This code was present (along with others) when I replaced the MAF at 44,000 miles, and was also present when I replaced the engine coolant temperature sensor at 46,000 miles. In both instances, I dealt with the most pertinent issue at the time, cleared all codes, then waited to see which would come back.

P0422 came back as a lone code at 47,000 miles, at which point I decided to clear it and wait to see how long it would take to reappear. The car managed a 600 mile drive through the Rockies, packed floor to ceiling with all my stuff, and a roof box with 180lbs of gear on top, without having the code return (despite poor fuel economy, wonder why... )

So now the code has returned at 51,000 miles. From researching, and the intermittent pattern of the code's appearance, I am quite certain my cat is slowly going bad. I have read that you can use a spark plug anti-fouler to trick the secondary 02 sensor into reading an acceptable emission content (and I did this to my previous car when I replaced the OEM header containing a cat with an aftermarket catless header).

The main question is: are there/what are the potential negative effects of going with the "trick the sensor" option vs. replacing the cat? I don't care about emissions; I just moved to Alberta where they don't really have emissions testing, and don't even get me started on climate change. My concerns lay around fuel economy, performance, and longevity of the car.

PS I have the torque pro app for android to connect to my ECU so I am able to check MAF, boost, air/fuel ratio, etc. readings, if anyone can lend information on how I can use those to determine the solution to my problem.

 

Have you considered replacing the sensor first? It's a lot easier and cheaper than replacing the cat.

 

from what I've read, it's very unlikely that the sensor is the culprit. It was mentioned by an audi tech on another site that Audi sensors throw their own code when they go bad; can anyone verify that?

 

Either sensor COULD throw a code for high/low heater element resistance or no activity detected, which would obviously be a good clue that the sensor was going bad. However, my secondary O2 sensor went bad and the only code I pulled was P0422. I replaced the sensor and it never came back. It's very possible it's just the sensor. Emissions aside, a bad O2 sensor is going to hurt your fuel mileage.

Since you don't need to pass emissions, I would just throw a test pipe and O2 sensor spacer on it and enjoy the extra torque.

 

Audi O2 sensors are typically just Bosch O2 sensors, which work like a normal O2 sensor. As CCA4 pointed out the heater inside a sensor can go bad, which would throw a fault code, but the sensing element itself can also go bad (which could show up as a No Activity code or a code like the one you have).

But only if it's a bad pre-cat O2 sensor post-cat sensor isn't used for fueling adjustments.

 

 

Thanks for the input guys, appreciate it. So by throwing a test pipe on you mean removing the cat and replacing it with just a pipe for improved flow, correct? Then tricking the secondary sensor with the spacer? Will this effect the exhaust note at all? Putting the catless header on my old car made thethe exhaust sound very tinny and raspy.

 

It'll be louder for sure, but the test pipe and anti-fouler should keep the CEL off. You'll likely gain a little power too.

 

It will be louder and more throaty, and you'll hear the turbo spool quite a bit more, which I happen to enjoy the sound of