Catalytic Converter on 1995 Audi A6 Quattro

My car used to rattle a lot especially on a cold winter day. The rattling sound which dissipates when the car warms up, was caused by a faulty catalytic converter (I was told my mechanic). Now that I moved to SoCal, the car rarely rattles. Do I still need to replace the converters especially if it passes the smog test?

Asked by for the 1995 Audi A6 Quattro
If engine performance is normal and your 1995 Audi A6 will pass its smog inspection I see no reason to replace the catalytic converter at this time. What the future holds, no one can say.
Passed the smog test. Thank you for the advise. Saved $2k+ for now. I know I will eventually need to be replace either or both. I have a follow-up question: in addition to failing the smog test, what other symptoms will my car show with a defective catalytic converter? If I wait on its replacement (since it passed the smog test), will it cause more & bigger repair expense later?
The answer to your question is ever evolving. On older vehicles, the common symptoms would be a lack of power due to the converter clogging (restricting the exhaust) and the rattling type noise you have noted. In later model years the catalytic converters have been moved closer and closer to the engine to improve efficiently. On some 2005 + vehicles the converter is now part of the exhaust manifold. If this type of converter begins to break apart, the debris can actually be sucked back into the engine causing internal damage.