P0179

P0179 is the high-input mirror of P0178 — the fuel composition sensor signal is above the upper threshold, typically a frequency >160 Hz or a stuck-high voltage near reference. High input almost always means either the signal wire is shorted to the 5V reference or to battery voltage, or the sensor's internal output stage has failed open and is letting the pull-up resistor inside the ECU dominate the reading. Workflow on a shop floor: (1) Disconnect the sensor connector and read the signal wire voltage at the ECU end with KOEO — if it stays pegged high with the sensor unplugged, the short is in the harness between the connector and the ECU; if it drops to zero or floats, the sensor itself is the fault. (2) Check the connector pins for green corrosion or pushed-back terminals — a signal pin touching the 5V reference pin in a contaminated connector produces exactly this symptom. (3) Confirm fuel-tank/sensor grounding — a missing or high-resistance ground at the sensor housing can elevate the signal reference and trigger high-input faults. The whole circuit-isolation test runs about 15 minutes with a DMM, and almost always points cleanly to either wiring or sensor without needing further teardown.

Vehicle-specific patterns: 2006-2011 GM trucks/SUVs FlexFuel commonly throw P0179 from a chafed harness where the signal wire shorts against the 5V reference at a frame-rail abrasion point — splice repair clears it. 2008-2012 Ford F-150 / Expedition FlexFuel throws P0179 when the sensor's internal output transistor fails open, which Ford service has tracked on certain build dates with replacement coverage under emissions warranty for many years. 2007-2014 Chevy Tahoe / Suburban FlexFuel will throw P0179 paired with P0178 on intermittent intervals when the connector is loose or corroded — same root cause, different snapshot of the same fault. 2009-2013 Dodge Ram FlexFuel V8 throws P0179 less commonly but typically traces to a pinched harness where the fuel-tank skid plate was reinstalled improperly. Repair cost $150-$400 — clean a corroded connector at the low end, replace the sensor at the high end.