Been getting a bank 1 lean code but no other faults. Watching the Sensor 1 output graph it clearly goes lean after a few minutes at very high speed. Sensor 2 continues to oscillate. Slow down and the sensor starts to oscillate again. The other noticeable difference is that Sensor 2 goes rich for a few seconds if you floor it (as you would expect), but sensor 1 just oscillates. I'm pretty sure the fuel pump feeds bank 2 first and then there is a transfer pipe to the bank 1 manifold, so if the pump can't supply the required flow rate the pressure will drop on bank 1 before bank 2. I'd be interested if anyone can dispute this theory.