1998 jetta- p0171

i have a 1998 jetta gls 5 speed. my car has been running a p0171 code for a month now. i have reset it 3 or 4 times and it keeps coming back. i have replaced the maf as well as upstream o2 sensor. i also replaced the clamps on my vacuum hose. what can the problem be? should i disconnect the battery to reset ecu?

That code is
System to lean bank 1

Most likely to be a Lambda sensor

i know. i changed the sensor and the maf. but the code is still coming up.