Been getting a lean Bank 1 code with no other codes. Suspect it is a sensor issue as it's quite hard to think of a failure mode that would only cause one bank to go lean and not also give a misfire or some other fault. Using Torque to graph the O2 sensor voltage it appears to be working fine at idle. The other day I left the display up on a trip on the motorway. After about 15 mins the trace went lean and although there was some change in voltage it never went into the rich region. At one point it came back to life for a while after I slowed down to change motorways but soon went lean again for the rest of the journey. Started the car the next day and it is working again. The only thing I can think of is a dodgy sensor connection. i don't know of a sensor failure mode that would give this kind of behaviour. I had recently changed this sensor because the heater circuit failed on the original one. Any other ideas?