I have been trying to locate the source of an oil leak that drips on the catalytic converter. I have looked at the output shaft seal and it is dry. I have cleaned the area and checked everything I can see from below the car, it only seems to leak after the car has been driven at highway speeds. Have run out of ideas. All fluid levels seem normal, even changed the transmission fluid. It looks like tranny fluid but can't find the source. I was told by the previous owner the transmission had been updated to the 2003 model tranny, mine is an 01, and had drain plugs installed. I have looked at them and they look dry. It's getting to the point I am about to remove the tranny.

 

Are you SURE it's dripping on the cats? I, and at least ione other A8 owner that I know of, had a problem with the rear-breather hose. The hose goes from the r/h valve cover (as you are looking at the motor) back to the middle-area, in the rear (up by the firewall). It's a PITA to get to, too.

 

that is easily checked ( oil or transmission fluid ) one of the has to be low? excalent link there A8AWD

 

How one checks the transmission oil on A8 (there are no drain plugs)?

 

I have the same problem after changing center differential fluid. I checked the level and re-tightened the fill and drain plug. I think I need to replace the seal (washer) on the drain plug. Mine is also an '01. The diff looks like a cone with cooling fins on the back of the tranny (right above the cats)

While at the GTA dyno day, someone threw the comment "smell those cats burn!". Turned out not to be the cats, but the fluid burning off them. You need an 8 or 10mm Allen key (with vise-grip clamped to the end) for the fill plug, no room for a socket. Dont open the drain untill you get the fill open. Good time to change the fluid, about 750mL - $10.

 

Maybe check the front and backside of the heads for minor engine oil leak. Leaky cam tensioner seals and valve cover gaskets can cause oil dripping onto cats, especially on the highway.