P1431

P1431 indicates the secondary fuel level sensor (Sensor 2, located in the secondary saddle of a two-sided saddle-tank, or on the passive side of a siphon-fed dual-pump setup) is reporting a value outside the rational range — typically the sensor's resistance is reading outside 40-250 ohms across the float sweep, or its rate of change disagrees with Sensor 1 by more than a calibrated delta over a fuel-level transition. Diagnose cheapest first: scan-tool both fuel level sensors and watch the raw voltage/percentage as the tank drains across a full driving cycle — the two should track within 10% of each other on a saddle-tank truck once fuel sloshes equalize. If Sensor 2 reads pinned at 0% or 100% while Sensor 1 sweeps normally, the secondary sender's float arm has separated from the rheostat (a documented saddle-tank failure) or the wiring to the secondary sender has rubbed through at the frame-rail clip. The expensive misdiagnosis: dropping the fuel tank ($600+ labor on a long-bed) when the actual fault is a $12 connector full of corroded brass — always pin-test the harness first.

Vehicle-specific patterns: 1999-2006 Silverado/Sierra 4.8L/5.3L/6.0L extended/crew cab with the side-saddle dual-tank or single-tank-dual-sender configuration is the canonical P1431 vehicle — the secondary sender's plastic float arm cracks at the pivot and floats off the rheostat (TSB 03-06-04-007 covers the redesigned float assembly). 2007-2013 Silverado/Sierra 5.3L sees P1431 from the siphon-jet pump assembly failing on the secondary side, which the PCM detects via the disagreeing sensor readings. 2003-2009 Hummer H2 6.0L gets P1431 from the same saddle-tank float failure. 2005-2009 Trailblazer EXT/Envoy XL with the larger fuel tank throws P1431 from a chafed sender harness at the body-mount crossmember. Estimated repair: $80 to $950.