[A4] How much for a catalytic converter for my A4?

[Brian Powell]

> I have never had to replace a converter in any of my cars (VW Golf --
> 290,000 km, 1988 Audi 90 -- 287,000 km, 1990 Audi 90, 310,000 km, Audi
> A4 -- 160,000 km, and a couple of Subarus that have gone over 250,000
> km).  Granted much (~70%) of my driving is highway.

But, you don't know where she lives. In places like Colorado where the  
temps go from sub-zero to 60+F in a day, catalytic converters can  
crumble regularly (especially if the car lives outside). I had to  
replace them on two Audis (both V6s, which each have two cats).  
Remember that internally cats are made of a ceramic mesh covered in  
platinum (or palladium or rhodium), and can be quite brittle. That  
said, unless you live in an environment that can go from cold to hot  
quickly, it shouldn't break unless there are other problems (you can  
destroy a cat by sending a lot of unburned fuel out of the exhaust,  
but your car would throw codes and run terribly if that happened).

> A bad converter will either be plugged up or blown apart.  If it is
> blown apart power loss will not be a problem and it will be LOUD.  If
> it is plugged up it will be consistent in terms of poor power.

Secondly, you are incorrect that a blown cat is loud. It won't make  
any difference in noise. Catalytic converters are for emissions only.  
What you will hear with a blown cat is a sort of popcorn type of sound  
with the windows down as you cruise at a relatively low speed. What  
you are hearing are the pieces of ceramic bouncing inside of the  
stainless steel container. An easy (but not certain) test to see if  
yours is bad is to crawl underneath and knock on the cat. If you hear  
pieces rattle around inside, it is shot.

So, what is the end result of all of this? Well, you can choose to  
leave it as is, or you can replace it. From Audi, they cost in the $1k  
range. I'm sure this is where your mechanic got the quote for the  
part. I bet the wholesale list price from Audi is around $700. If you  
buy a generic aftermarket, you'll need someone to cut the existing  
pipe and weld the new one in, just make sure they don't muck up the  
post-cat O2 sensor plug.

Short from taking it off of the car, you have no quantifiable way of  
really knowing it is shot.

As for your O2 sensors, you really should change them around 90k. O2  
sensors can continue to give readings that don't trigger CEL codes;  
however, their response becomes extremely poor. You can hook them to  
an oscilloscope and vary the throttle. You should see a nearly  
immediate and sharp signal. Usually (around 90k is a guideline) when  
they are no longer as responsive, it will be a very gradual curve as  
the sensor responds to the O2 reading. This causes the engine mixture  
to be slightly off. You should change them, but you probably won't  
notice much difference in the car. You might see a couple of percent  
improvement in mileage, but you really have to be taking careful  
calculations to notice a couple of percent.