Summary
P0171 means the engine's air-fuel mixture on Bank 1 is too lean — too much air or not enough fuel. The most common causes are vacuum leaks (35%), a dirty or failing mass air flow (MAF) sensor (25%), and weak fuel delivery (20%). Start by inspecting vacuum hoses and cleaning the MAF sensor ($8 cleaner) before replacing parts — those two checks resolve over half of P0171 cases.
Severity: Moderate
Safe to drive: Yes, short distances — prolonged lean running damages catalytic converters
Repair cost: $8–$600 depending on cause
DIY difficulty: Easy to Moderate
What does P0171 mean?
Your engine needs a precise 14.7:1 ratio of air to fuel for efficient combustion (the stoichiometric ratio). The PCM constantly adjusts fuel injector pulse width to maintain this balance, using O2 sensor feedback. It tracks these adjustments as fuel trims — short-term fuel trim (STFT) for immediate corrections and long-term fuel trim (LTFT) for persistent patterns.
When the PCM has to add more than ~20-25% extra fuel consistently to compensate for a lean condition, it gives up and sets P0171. The code means "I've been adding fuel to Bank 1 for a long time and I still can't get the mixture right."
Bank 1 is the side of the engine with cylinder 1. If you also have P0174 (Bank 2 lean), the problem is likely in a shared system — intake, MAF sensor, or fuel delivery — rather than something bank-specific like an individual injector.