What causes a po171 too lean bank 1 code on a 1999 kia sephia 1.8 dohc?

A P0171 code indicating "Too Lean Bank 1" on a 1999 Kia Sephia 1.8 DOHC typically results from an imbalance in the air-fuel mixture. Common causes include a vacuum leak, a malfunctioning mass air flow (MAF) sensor, clogged fuel injectors, or a failing fuel pump. Additionally, a dirty or faulty oxygen sensor can incorrectly signal the engine control unit, leading to a lean condition. Addressing these issues usually involves inspecting and repairing vacuum lines, testing fuel delivery components, and cleaning or replacing sensors as needed.