P0171

P0171 is the canonical "lean" code on bank 1 — too much air, not enough fuel. The ECU has corrected fuel trim past its allowable +25% range trying to compensate. Diagnosis order on a shop floor: (1) Smoke-test the intake to find vacuum leaks — by far the most common cause, especially on engines past 80k miles where the rubber boots to the throttle body or PCV valve crack with age. (2) Check the MAF sensor reading at idle (should be 2-4 g/sec on a typical 2.0-2.4L 4-cylinder, 5-8 g/sec on a V6) — a dirty or failing MAF reports artificially low airflow, which makes the ECU under-inject fuel, which makes the O2 sensor cry foul. (3) Fuel pressure test at the rail with a mechanical gauge — most port-injection systems run 35-55 psi at idle; below spec means weak pump, clogged filter, or failing regulator.

Vehicle-specific patterns common in shop work: 2004-2014 Ford F-150 / Explorer / Mustang with the 4.6L or 5.4L Modular V8 often throws P0171 from intake-manifold cracks at the rear-driver-side corner — Ford issued multiple TSBs and many engines need the upgraded composite manifold. 2007-2014 GM 3.6L V6 (Camaro, Equinox, Acadia, Traverse) often throws P0171 from a failing PCV valve in the cam cover causing a manufactured vacuum leak — $40 part, 30-min repair. 2002-2010 Volkswagen / Audi 2.0T FSI/TSI throws P0171 from intake-manifold gasket failure ~80k miles AND PCV diaphragm failure on the valve cover at ~100k miles — both are common, sometimes simultaneously. 2008-2015 BMW N51/N52/N54/N55 inline-6 throws P0171 from valve-cover gasket leak (oil drips onto the exhaust manifold AND introduces a vacuum leak as the gasket degrades) — replace with VCG kit including bolts.