w211 e200 issue after 5k RPM

[atsa2013]

Hi all...I have an issue with my e200 kompressor (2006 184bhp) car is even less responsive and starts shaking once I go over 5k rpm or give it kickdown there is a noticeable lack of power.when I turn off the engine and start up again it runs fine aslong as I don't go past 5k RPM also as a result error p0171 popped up (system too lean bank 1) ... All this caused fuel consumption to increased (13l lasted for 75km )along with loss of power.... Any ideas what's going on??? Thanks