Eugene, I'd be guessing but it sounds like it's be bank 2 and post cat sensor...
But, 02 sensor codes often seem to come about because of other things pretty often...though with only one 02 code (assuming the heater is built into sensor) that might be more reliable.
I do know that cats on an older 750 have heaters independent of the sensor, but don't recall that on an e36.
Performing at the Comedy Cove--595 Morris Ave. Springfield NJ reservations 973 376-3840
A recent set
dmax on the radio every Wednesday 7-9 p.m. NYC time. Call in! (661) 449-9340 watsonnwatson.com