Thanks for response, MB DOC. The dealer did the catalyst campaign (checking for cracks - that's all that I was told was involved) in October 05, at which time they installed the new B2S2 (LR) O2 sensor, which is now the one throwing 161 code. I checked the wiring and this sensor is getting voltage (13.4) so it seems fine. Maybe I should replace the front O2, maybe as it ages it is affecting the rear one?