Ford: DTCs and service repair, p0171 system too lean bank 1, o2 sensors

Question
Hello Ragu, I have a 1999 F-150.  I purchased a DTC reader because my service engine light came on.  Unfortunately for me a reliable service center is about 100 miles away (I live in the country). Anyway, there are 2 codes showing up...  P0171 (System too lean - bank 1) and P1131 (Lack of HO2S11Switches - Sensor indicates lean).  With the exception of low mpg - around 15.0 which has been constant since I purchased the truck new in 10-98, my 5.0L engine seems to be running just fine.  These DTCs have been set for approximately 1 year.  Is there anything that I can do to adjust for the leanness in bank 1 (which one is that?) and tale care of these DTCs?.  What would be done at a Ford Service Center to make these adjustments and take care of the problem? I have had very good performance with my truck that now has a little of 168k miles.  Once I had to replace a defective injector coil on #4 coil and last year replaced the throttle air by-pass valve. Two years ago replaced the EGR Pressure Valve.  Nothing else has been required. Thanking you in advance

Answer
a lean code doesn't always mean o2 sensor is bad you need to ck for vacumn leaks,clog air filter dirty throttle body clog pcv valve ,inj flush.Bank 1 is the one on the passenger side. now if i had 160,000 miles on the truck and the o2 sensors never been change then maybe it time to change the dealer says they should be change every 30,000 miles to keep up good gas miles and normaly thats all there good for .