Before stripping can you get access to fuel trims bank 1 bank 2 or Lamba output voltage and compare and check heater circuit on both.
Also worth checking a wiring diagram just in case sensors are crossed over by mistake either side to side or front o2 and rear 02(if loom allows, you could just swap anyway and try)
Best of luck, I have similar fault on Astra 1.4 (HC1500) but that is loom/ECU No funds to continue as local fast fit used up all his dough..