I had the front O2 sensors changed with the Bosch replacements in December, when I had the transmission serviced.

My engine was running a little rough last week, so I bought new plugs for it.
When I replaced the plugs, the right bank (cylinders 1,2,3), were moist, black, and sooty from running rich. The left bank (cylinders 4,5,6) were white and dry from running lean.

The car runs fine after putting in the new plugs, and cleaning the MAF sensor.

I pulled plugs 1 & 4 today to check them, and the engine is still running 1/2 rich, and 1/2 lean (the same halves .

 

You may want to replace the other side as well. O2 sensors do have life expectancy. If one goes, then the other will soon follow.

 

I did both of the fronts, but I do have a new pair of rear sensors. I'm going to put them in later this week.
As I understand it, the front sensors affect the fuel mixture, and the rear sensors don't. Anyone know more about the function of the front vs. the rear O2 sensors?

 

Rear ones are for checking the converter efficiency. They don't affect the fuel mixture. I misread your post thinking that you replaced one side only. There is a change that you got a bunk one. It happened to me. Which one is which? Don't know. You can run through the o2 sensor tests if you have vagcom.

 

Judging by the age and condition of the plugs, the uneven fuel mixture problem was there before I replaced the front O2 sensors.
I do need a vagcom, and I will get one as soon as I see one for a decent price.

 

ebay ones actually does wonders. If you look around the web, there is a crack floating around somewhere.

 

Which of the two fuel mixtures is more normal for this engine, lean, or rich?

 

And a much delayed follow up. I did get a vagcom, and replace the rear O2 sensors. The right side fuel mixture is still rich, especially on startup. and Bank 1 has an error code of Efficiency Below Threshold.

 

PS, I have replaced the timing belt recently, and the car runs great. Smooth at idle, accelerates well, and cruises with 22-24mpg.