P0174

P0174 is the bank-2 mirror of P0171 — the ECU has run long-term fuel trim on bank 2 past +25% trying to add fuel and the upstream O2 sensor still reads lean. Because the code is bank-specific, the diagnostic narrows fast: anything that affects both banks simultaneously (low fuel pressure, dirty MAF, failed pre-cat O2 reference) will throw P0171 AND P0174 together; if only P0174 is set, the fault is mechanically isolated to bank 2's intake or exhaust path. Cheapest-first order on a shop floor: smoke-test the intake with the smoke wand fed into the brake booster line or PCV port and watch the bank-2 manifold runners, intake gaskets, and the bank-2 PCV/breather hose route — about 70% of single-bank lean codes are vacuum leaks on the runner-gasket or a cracked rubber elbow specific to that bank. Then check for an exhaust leak between the bank-2 manifold and the upstream O2 sensor (a leak there pulls in fresh atmospheric air, makes the sensor read lean, ECU dumps in fuel that never gets used). Finally, swap bank-1 and bank-2 upstream O2 sensors and see if the code follows the sensor — if it does, it's a bad O2; if it stays on bank 2, the problem is mechanical. STFT bouncing >+15% under steady-state cruise is the live-data signature.

Vehicle-specific patterns: 2004-2010 Ford F-150 / Expedition / Navigator with the 5.4L 3-valve Triton throws P0174 alone from a cracked composite intake manifold on the passenger (bank 2) side near the EGR port — Ford issued multiple TSBs and the upgraded manifold with aluminum coolant crossover is the durable fix ($350-$500 part, $700-$1,100 installed). 2005-2010 Chrysler 300 / Charger / Magnum with the 2.7L or 3.5L V6 commonly throws P0174 from cracked intake-manifold plenum gaskets on the rear (bank 2) bank. 2000-2007 BMW E46 / E39 / X5 with the M54 inline-6 throws P0174 from disintegrating DISA valve O-rings and crankcase-vent (CCV) diaphragm failure that vents directly into the bank-2 intake path. 2006-2012 GM trucks with the Vortec 5.3L throw P0174 from a failed manifold-absolute-pressure-style intake gasket on the bank-2 side around 100k miles. Repair cost typically $50-$600: a cracked vacuum hose is the low end, full intake-manifold replacement on the 5.4L Triton is the high end.