My X is throwing the code Code P0141 - O2 Sensor pre-heater failure



My questions are:
1) do I need to fix this immediately?
2) I think the part number is 11781433940 so a post-cat sensor (again I think). What is a good price for these? Buying off ebay a bad idea?
3) How do I determine which cat assembly to work on?

Thanks,
Repair noobie