Engine Performance problem
1999 Ford Explorer 6 cyl Four Wheel Drive Automatic 124,00 miles

My check engine light came on. I had it scan and the code return PO171 which means my system is too lean.
What does that mean, and how is this problem repaired?

means that the oxygen sensor on bank one has detected to much oxygen in the air fuel mixture.

first thign to do is disconnect and remove the MAF sensor, it is located right after air box before throttle body.

clean off the contacts with qd contact, dont touch with your hands as they break easily.

if that is clean and tests out ok then chances are you have a vaccum leak somewhere after the sensor causing unmeasured air to get in.

either that or you have an exhaust leak on bank1