I am putting this question out there to see what I should do.. A little background info first. I live in California and my mother comes to stay here between here places in NY, FL and Italy. She is a sprite 74 and she has a 1999 Audi A6 Quattro 2.8L AHA Motor here in California to use when she is around and just a spare vehicle for family to borrow if they need it. She has only spent about 6 months total in California in past 10 years as her boyfriend lives in NY. Not really caring that my brother had the car for most of the past 6 years, I was surprised to see the car reappear at my house a few months ago (I have maintained a few rooms for my mothers use when she is in California) and thought nothing of it. A few days later when I was moving vehicles around so I can park her car where I used to leave it I was appalled at what I found. Bad paint, torn intererior, dead battery and when I finally got it started 310000 miles on it. Multiple Check engine lights, coolant leaks, no AC, no fans, brake replacement indicators on , cracked windshield, bad control arms and just plain worn out. Last I new it had around 135000 and was pristine. (Car purchased in 2002 w/ 27000 on it.) Needless to say a lot of screaming and yelling occurred as I started to as up what it would take in parts alone to make the vehicle safe for my mother and usable as a spare vehicle as my brother whined about the transmission, tires and catalytic converters he bought with irregardless of the fact he put more than 150000 miles on the car. Long story short I got him to agree to paying out a total of 2500 which based by miles driven is then best per mile cost anyone I can think of has paid.

I decided to replace the Audi A6 with another one with less miles as my mother likes the car and feels safe in it. She has a 2016 A6 in NY and uses that in FL also, I landed up getting a 2000 A6 Quattro ATQ with 150000 on it for 1500 which needed some work but the last timing belt was done at 135000 after a premature water pump failure so at least it shouldn't require much maintenance to begin with and all the stuff that usually fails seem to have been replaced, (MMI ABS Controller, Cooling System, Power Steering, Control Arms, AC System) A few issues like sunroof frame, cracked taillights, bad window regulator broken visors seat switches etc. I can just swap with her original car and have a pretty decent and reliable car.

Now here is my main question, The catalytic converters are known issues with these model cars and living in California I have to smog this car every two years. My brother got a steal on two OEM Audi Catalytic Converters about 3 years ago and I would lime to install them on the 2000 A6. The part lists different cats for the 1998-1999 and the 2000-2001 A6's I guess because one is an AHA motor and the other is an ATQ motor. I have removed both cats from both vehicles and the 1999 cats seem to fit the 2000 fine. I also know they use different O2 sensors. I haven't installed either the original or the older 1999 cat in the 2000 yet but am wondering if it will work correctly or not. The cats shouldn't matter because they are physically doing their job but I think I should use the 2000 O2 sensors with the 1999 cats in the 2000 A6. Anyone have any idea or thoughts on this, I hate to get rid of the 1999 A6 cats and then need a set of cats next year when I could have used these but if they won't work correctly then thats what I'll do or should I just try and see what happens. Any input is appreciated

Thanks

 

The cats on the ATQ aren't the real issue; it's the ECU, which has too high of an efficiency threshold...not that you can do anything about that.

The ATQ uses a pre-cat, and measures only the efficiency of those pre-cats. You might be able to extend the wiring of the O2 sensors to work with the AHA cats, but you are probably better off buying cats that fit the ATQ and will pass CA smog without issue.