code p0171 2013 subaru forester cel and cruise light on

Recently my little soobie's cel and cruise light came on.  I have noticed over
last month that the engine has been knocking esp at lower speeds and when
under a load (accelerating) and has had slower pick up..  SInce the light
came on it stumbles (once at about 35 mph) but mostly at  lower speeds and
even shutters at an idle, like it wants to die.  I only drove a short distance
with cel and cruise light on, so I can't give much more information on how
she behaves since the light came on.   When I hooked up my diagnostic tool
it read  OBD code   P0171            Looking for advice.