97 Z34 P0420 Code

Keep getting the P0420 Code, Catalyst Efficiency Below Threshold (Bank 1)
Anyone know where I should start? I hate to replace parts that aren't defective, I've done this in the past, gets real expensive. It has 78,000 on it, new timing belt, alternator, starter, numerous other parts replaced that I can get to. could it be o2 sensor?
Exhaust system doesn't seem to be putting out as much (place hand at tail pipe not much there)

what ruins a catalyst? Too much fuel and not enough spark.The ecm can be flashed and a new sensor may not be enough,the 420 is from updated emissions compliance.
federally mandated to keep the ozone.your catylist is no longer able to process efficiently enough to pass.The dealer can replace this under warranty before 80'000 mi.