1997 Ford Taurus low effiecenty catalyst bank two

We replaced the cataylitic converters on october of 07 because of low effiency in bank two. This past spring we replaced the whole assembly y-pipe again due to low effiency of catayalist in bank two again. In August we repalced the H2os in bank two after the cat and the check engine lite is back on again. The cats were replaced under warranty and the shop was told by the supplier that the cat was bad. Is it possibe that the cat is bad in the replaced unit as well. It seems to go about three months and the light comes on. Two of the four H2OS have been replaced one a couple of years ago. That one is the front one that can be seen when looking down at the front exhaust manifold at the front of the engine.Any ideas would be helpful.