2002 Subaru Legacy O2 Bank 1 Sensor 1 code

P0130 recently - I've had a P0420 for years due to an aftermarket catalytic
converter, but I've never had an O2 sensor related code.  I've read up on this
and saw that the voltage should be cycling between around 0.1V and 0.9V,
and I saw that the sensor labeled "O2B1S1," the one tripping the code, was
reading around 2.5-2.6V. Fuel economy seems to be off and occasionally
while the car is idling and warm the engine will sputter but never die, so I
think I'm running at a very rich mixture right now. Rear O2 sensor is
consistently around 0.9V. Inspecting the wiring of the O2 sensor itself, I didn't
notice any obvious corrosion, but the connector that connects the O2 sensor
to the rest of the wiring was damp. Are there any other numbers I should
look at to diagnose this problem properly? Is it just time to get a new front O2
sensor?
Thanks