P0171 — Fault Tree

P0171 — System too lean (bank 1)

The engine is getting more air than the computer thinks it is, so it is adding fuel to compensate and has run out of adjustment range. Almost always unmetered air getting in after the sensor.

System: Powertrain
Definition: Generic (SAE-defined, all manufacturers)
Subsystem: Fuel and air metering
Severity: moderate

Causes, by prior probability

Cause	Prior	Repair
Vacuum leak (intake gasket, PCV, hose) Smoke test the intake. Trims that improve at high RPM point to a leak, since the leak is a smaller fraction of total flow.	38%	$194
Dirty or failing mass air flow sensor Compare MAF g/s against expected for displacement at idle. Clean before replacing.	26%	$153
Weak fuel pump or clogged filter Fuel rail pressure under load, not at idle. A weak pump holds pressure at idle and collapses under demand.	14%	$437
Leaking or restricted fuel injector Balance test across injectors; a single lean bank with one dead injector shows as cylinder-specific misfire.	12%	$415
Failing upstream O2 sensor A biased-lean sensor commands fuel it does not need. Check against a known-good scan value.	10%	$231

These are priors from the code alone. A scan's freeze frame reorders them, which is the entire point of capturing it.