Number68

also, yes, a faulty o2 sensor can cause a problem with the cats, if the engine runs too lean the cat will run too cool, thus not effectively converting everything

type44

Really? I would have guessed the lean condition that raises combustion temps and nox levels would cause the cats to get hotter, but produce more nox than it can convert? But I have been known to be wrong 1 or 1,000 times, lol.

Number68

Yes, you would be correct in that respect, running lean CAN and WILL create higher temps, thus high levels of NOx. SO, it could be a problem with vacuum leaks, ****ty injectors, bad timing etc causing the car to run lean, and creating more NOx.

Lean exhaust also causes the cats to run MUCH cooler, and not be nearly as effective in converting the bad stuff(great technical term, eh?)

generally speaking, at idle there is no measurable amount of NOx, but when the engine is under load is when it is produced, which is why egr valves work (they open depending on the level of vacuum or exhaust pressure, typically)

It could be a couple different things, like I said. on a newer car, I'd start at the EGR valve, but as I said earlier, these cars don't have one. The only real emissions thing that these cars have is the O2 sensor, which because it's narrowband only shows hey, it's lean, or rich. Idealy it bounces back and forth to keep the cat running the most efficiently.

He said already that he checked the vac lines, spark plugs, etc... assuming there are no vac leaks, and the timing and everything is good, it leads me to believe the O2 sensor is giving a faulty reading, heck, it could be one of the temp sensors too now that I'm thinking about it....

Number68

wow, I'm making a lot of posts on this thread...

Quick question, you don't by any chance have the old plugs still do you? I'd love to see a pic of the electrode to determine if the car is actually running lean....

OverG

Wow, thanks for all of the discussion and tips guys. I'll answer all the questions I can here. o2 sensor is new, no change with it, so old one must have been okay. No vacuum leaks unless it's somewhere I cannot access or a manifold gasket or something, although there is no sound to indicate this and a spray on all hoses and fittings with a water bottle while car is running gives no hiccup or stumble. I tossed the old plugs, but they were dry and darkish gray in color, uniform condition on all five.
I've got the car for sale, and am turning my attention to my Eurovan (just purchased), which barely failed HC and NOX, so all of your info here is going to help with that. It's been sitting for months and I'm sure a tune up will help it.

The only item left with the Audi that I can imagine is the cat. Which is welded in and looks pretty new (maybe it's a cheapo and not a good one), but I'm not going to invest the time and money if I can find a buyer in a neighboring town where none of this is an issue. It's a great car, but I'm done with it. Thank you all very much, the info helped and will continue to help with my camper's 5 cylinder VW/Audi engine!

OverG

Could not eek out that final few points, car sold to next county (no emissions testing there, think I'll look for property!), first buyer to look at it paid and drove her away, straight to the DMV for plates, easy as 1,2,3. Really think I'll move now!

Thanks to all of you for the help. Currently looking for replacement, and got my eye on a nice 4000CSQ. I'll be back...

Greg