I had the joys of this one today and managed to sort it out but it did throw a couple of curve balls.
the first being mot fail high emissions on the fail sheet almost 10percent.
p0170 system lean. leak tested inlet and exhaust no leak. the garage had already fitted new cat ant lambda sensors tested the ground of the o2 sensor upstream and found a -.03v reading (below ground??)
tested sensor output 10Htz and switching cleanly
engine earth same a minus reading even with an extra earth clamped to battery.
mass airflow tested OK
cleaned out the dirty throttle body and checked live data to confirm ect and air temp sensors
found no problems to suggest engine lean (fuel trims normal)
tested the emissions and found to be OK
printed out emission report (mot station must have problem with gas analiser)

but i don't understand where the minus below ground signal derives itself from