How do you troubleshoot P0401 and P0171 obdII fault codes on a 1997 Buick lesabre?

p0401 is lack of egr flow

p0171 is bank 1 lean

So we have two problems that are commonly related, often accompanied by high idle that will not return to normal.

With out high idle problem you may have a pluged EGR or Vacume leak at the EGR system.

THe High idle is definately a clue that you have leak in the EGR system.