P0151

P0151 is bank 2 upstream signal stuck LOW — same diagnostic as P0131 but on the other bank. Scan fuel trims first: if B2 STFT + LTFT are pegged at +20 to +25%, the engine is really running lean on bank 2 and the sensor is correctly reporting it. That points to a bank-2-specific vacuum leak (intake manifold runner gasket on a V6, valve cover gasket on bank 2 only, cracked PCV hose routed to bank 2 intake), a bank-2 fuel delivery issue (one injector on bank 2 partially clogged, or a cracked fuel rail), or a bank-2 exhaust leak upstream of the sensor introducing fresh air. If fuel trims are normal, the signal is being pulled low by wiring — backprobe and check for shorts to ground. Critically: confirm bank assignment from the FSM before you smoke-test or replace anything.

Vehicle-specific patterns: 2005-2010 Ford F-150 5.4L 3V — bank 2 is passenger side; throws P0151 from cracked exhaust manifolds on passenger side specifically (the well-known broken-manifold-stud TSB). 2007-2014 GM 3.6L LFX/LLT — transverse, bank 2 is front; throws P0151 from a torn front intake-manifold-runner gasket. 2008-2014 Dodge Avenger / Journey 3.5L SOHC — sets P0151 when the front (bank 2 transverse) intake plenum gasket fails. 2009-2016 Audi A4/A6 3.0T supercharged — sets P0151 from a leaking T-fitting on the bank 2 PCV hose. Cost band: $200-$500 for sensor; $300-$1,200 if the actual cause is a manifold or VCG leak.