Durametric fault code question

[sfoster13]

Getting these two codes:

PO139 - Fault Code 17 - Sensor ageing behind cat converter cylinder (1-3)
PO140 - Fault Code 12 - O2 sensor behind cat converter cylinder (4-6)

Car is a 2002 S

These are just for bad 02 sensors, right? Nothing about the cat itself? Just replace the two sensors? And does behind mean before or after the cat?

Thanks for the help