P0171 — "System Too Lean (Bank 1)" — is one of the top five most common OBD-II codes. It means your engine's air-to-fuel ratio on the driver's side bank is too lean: too much air, not enough fuel.
The four most common causes (in order of likelihood)
1. Vacuum leak — most common, cheapest fix ($0–$250)
Any crack in an intake hose, a loose clamp, or a failed intake manifold gasket lets unmetered air into the engine. Spray carburetor cleaner around intake hoses with the engine running — if the idle smooths momentarily, you found the leak.
2. Dirty or failing MAF sensor ($10–$300)
The Mass Air Flow sensor measures air entering the engine. A dirty sensor under-reports airflow. Try cleaning it first with MAF cleaner spray ($10). If the code returns, the sensor may need replacement ($100–$300).
3. Clogged fuel injectors ($100–$400)
Injectors that can't deliver adequate fuel cause lean conditions. A fuel injector cleaning service ($100–$150) often resolves this.
4. Weak fuel pump or clogged filter ($50–$900)
A failing fuel pump starves injectors of fuel under load. This typically shows up as lean at highway speeds. Also check P0087 if it appears alongside P0171.
Is it safe to drive with P0171?
Short term yes — but lean conditions stress engine components. Fix within 1–2 weeks. See the full P0171 code page for complete diagnosis steps.