P0171 - System to Lean (Bank 1) The Adaptive Fuel Strategy continuously monitors fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit. For lean and rich DTCs:
Excessive fuel pressure. i doubt it on a focus
Leaking/contaminated fuel injectors. mayby
Leaking fuel pressure regulator. we aint got one
Low fuel pressure or running out of fuel. could be, any other problems?
Vapor recovery system. nah
Air leaks after the MAF. do you have an air leak after the maf?
Vacuum Leaks. do you have a vacuume leak?
PCV system. mayby your pcv need to be replaced? take it out and shake it. if it rattles its good. i think
Improperly seated engine oil dipstick. fix dat [censored] man
Leaking gasket. doubt it
Stuck EGR valve. nah
Leaking diaphragm or EVR. huh?
Oil overfill. check it foo
Cam timing. did you f*** with your cams?
Cylinder compression. does it run smoth? take a compression test
Exhaust leaks before or near the HO2Ss. do you have an exhaust leak?
A SHRTFT-1,2 PID value between -25% to +35% and a LONGFT-1,2 PID value between -35% to +35% is acceptable. Reading beyond these values indicate a failure.
dont worry about this unless you have a wds or scan tool
or just beleive your friendly ford tech and replace that o2. but i would sugest checking that stuff mentioned as well