P0171 1997 Ford F150 -

Question

Where is this sensor located

Answer 1

P0171 Fuel trim too lean bank 1

This is not a sensor but a condition, of fuel and air ratio, when there is too much air you can develop a lean condition, lean condition are usually unmeter vacuum that gets past the MAF (mass air flow), look for disconneted  vacuum lines and listen for vacuum leaks especially at the PCV hose and valve.