Car is 97 Z28 A4 and is stock, 112k miles. My question is how can I tell if its time to replace my catalytic converts. Car is running just fine.

 

Your OBD-II 1997 has "after-cat" O2 sensors to measure the efficiency of the cats. If you aren't getting an SES light and a code, the cats are probably performing within specs.

The other area of concern would be the cats plugging up. This would cause a lack of power, and possibly glowing exhaust manifolds. If your car ir performing at the same level as it was in the past, the cats are probably fine. If you want to have them checked, it is possible to measure the backpressure in the exhaust system by tapping into the AIR connections. But I don't know the specs for allowable backpressure.

 

Thanks Injuneer

 

you can also get a little gun thing (forget the name) to measure heat before and after the cat to tell you if it is clogged up. most muffler shops will do this for free.