Ford 2000 Ranger 4L automatic 2 WD, 138 mile

The check enging light turned on with no performance or noise issues noted. I read the trouble code & it was 0171 (lean bank one). I reset the light & it did not return until about 2 weeks later & this happened after I accelerated very quicky from a dead stop due to a traffic issue. I checked the code after the light came back on & it was again code 0171. I could find no reason for the code so I reset it & after about 2 weeks it can on again this morning. Do you have any ideas before I take it to a repair shop? I see no performance, noise, or other issues but I didn't want to do more damage by ignoring this.