Please help 
I have replaced the Valvetronic sensor, but it hasn't helped. Had a look at SBSR on the driver's side, behind carpet to the right/rear of the seat. It's a little blue plastic device, with potted electronics, not the bigger metal box of older Z4's. Took the fuel pump controller out and opened it up. There is no sign of damp, or corrosion on/around either units. No sign of dry solder joints. The car's battery is in good condition and is fully charged. Engine runs smoothly, albeit fuel consumption is increased and performance reduced. I am out of ideas
2008 2.5si (N52 engine).