P1151

P1151 is the Bank 2 Sensor 1 mirror of P1131 — the upstream O2 on the driver side of most Ford V8s is reporting a persistent lean condition with insufficient rich/lean switching. Same fundamental issue as P1131, opposite bank, and on Modular 4.6L/5.4L engines a P1131 plus P1151 set together almost always points to a shared vacuum source rather than two simultaneous O2 sensor failures. Cheapest-first ladder: read LTFT Bank 2 — above plus-10 percent confirms real lean (spec is plus or minus 8 percent at idle); compare to LTFT Bank 1. If both banks read plus-12 percent or higher equally, the leak is on a shared intake source like the upper plenum gasket, the throttle body gasket, the brake booster hose, or the EVAP purge solenoid stuck open. If only Bank 2 is lean, smoke-test driver-side intake runners and the rear PCV vacuum tee that feeds the brake booster on 5.4L Tritons. The expensive misdiagnosis is replacing the driver-side O2 and exhaust manifold based on the lean code when the actual fault is a $25 brake booster hose feeding both banks unmetered air.

Vehicle-specific patterns: 1999-2004 Ford F-150/Expedition 5.4L 2V trucks crack the plastic intake manifold on the driver side around the coolant crossover — Ford TSB 06-26-3 covers the plastic-to-aluminum intake update. 1996-2004 Ford Mustang GT 4.6L 2V cars suffer driver-side valve cover gasket failure that lets oil onto the exhaust manifold and cooks the upstream Bank 2 sensor. 2004-2008 F-150 5.4L 3V trucks suffer driver-side spark plug blowout exactly like Bank 1, often setting P1151 with cylinder 5 or 7 misfire codes. 1997-2003 Ford E-150/E-250 Econoline 4.6L vans crack the rear-of-engine PCV vacuum tee buried under the doghouse, leaning out Bank 2 first. Estimated repair: $25 to $1,400.