Question

I have a 2004 Toyota Sienna with engine code P0171, what could be the problem?

Answer 1

P0171 Fuel trim lean Bank 1

When the fuel trim is lean there is more air to fuel ratio, it always a good procedure to visually inspect for vacuum lines being connected and no torn or rupture lines before looking into a mechanical condition.

 Here vacuum line being disconnected lead to a lean fuel trim.

Answer 2

P0171 TOYOTA Code - Air/Fuel Mixture System Lean Bank 1
Read more: https://www.autocodes.com/p0171_toyota.html