left bank lean mixture -MAF filter cleaned -problem returned -code P0171

Maybe a bad O2 sensor.

If no vacuum leaks detected. Hook a scanner to it and go to data and monitor load percentage. Take out on an area you can run it. Get in open stretch and go wot. If load percentage does not reach in the 90% or better. You have a bad mass air flow sensor

Possible causes: Intake air leaks, Faulty front heated oxygen sensor, Ignition misfiring, Faulty fuel injectors, Exhaust gas leaks, Incorrect fuel pressure, Lack of fuel, Faulty Mass Air Flow (MAF) sensor, Incorrect Positive Crankcase Ventilation (PCV) hose connection...If the P0171 is combined with the P0174 code, it's very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replace the air filter and clean the air flow meter. If the problem persists the front oxygen (O2) sensor may need to be replaced.