The notorious P0420 code ... again

I've got a 2002 Subaru Outback Wasgon. 2.5 with 122K+ miles. I've had the CEL light (code P0420) on for a while. More than a year. Occasionally it went off. There aren't any performance problems. After much research I replaced the PCV valve and the front O2 sensor. As soon as I installed the sensor, the CEL light went off for exactly 66 miles. The idle in gear seemed a bit smoother. I used a Bosch sensor, which claimed to be OE. Is Bosch OE or is it possible that the sensor is already bad? Or should I replace the rear sensor? I've taken it to the mechanic and they want to replace the cat converter, based solely on the code, which seems like a bad idea to me especially at the price. Do I need to take it to a shop and have the voltage of the O2 sensors checked? Any help is greatly appreciated.

If you've replaced the front O2 sensors then it is probably the catalytic converters. The best way to check is to graph the front and rear O2 sensors. If the catalytic converters are shot, they will match. If not then the front will be up/down/up/down and the rear will be a fairly straight line.