P2243

P2243 means the ECM has lost the 5V reference voltage feeding the wideband (air/fuel ratio) sensor on bank 1 sensor 1 — the upstream sensor that drives the fuel-trim loop. Modern wideband sensors are not the simple narrowband switching sensors that throw P0130-series codes; they require a regulated 5V reference (the same VREF rail that often feeds the MAP, TPS, and pedal-position sensors) plus a heater circuit and a pump-current return. When P2243 sets, it usually means the VREF leg is open, shorted to ground, or drooping below spec — not that the sensor element itself is dead. Cheapest-first ladder: back-probe the 5V VREF pin at the upstream O2 connector key-on-engine-off, must read 4.9-5.1V steady; anything below 4.5V or above 5.3V is a regulator or shared-circuit fault, anything reading 0V is an open circuit between the ECM and the connector. Check the harness ground reference at the same connector (under 0.1V drop to battery negative). Wiggle-test the engine harness with the scanner connected and the freeze-frame loaded — most P2243 calls in shop work trace to a chafed or corroded splice in the engine-harness branch, not the sensor itself. If VREF is in spec at the connector but the sensor's signal still reports open, swap in a known-good sensor before condemning the ECM. Before condemning the wideband sensor on a P2243, ALWAYS verify the 5V reference at the connector — replacing a $250-$450 wideband sensor on a vehicle that actually has a chafed VREF wire or a failed ECM driver just sets the code again on the next drive cycle.

Vehicle-specific patterns: 2010-2016 Ford 3.5L EcoBoost (F-150, Flex, Taurus SHO, Explorer Sport) is the dominant P2243 platform in shop work — a known engine-harness chafe point at the engine-harness-to-bulkhead connector corrodes the shared 5V VREF, and Ford issued a TSB covering harness inspection and repair on early 3.5 EB VINs; check service-history coverage before quoting a sensor. 2012-2018 Ford 2.7L EcoBoost shares the same harness-family weakness. 2008-2014 Subaru EJ255/EJ257 (WRX, Forester XT, Legacy GT) throws P2243 from a brittle wideband connector at the up-pipe where exhaust heat cooks the harness insulation. 2007-2013 BMW N54/N55 (335i, 535i, X3 35i) throws P2243 from a failed DME 5V driver on high-mileage cars — the same VREF rail also feeds the MAP, so a P2243 paired with P0107/P0108 points at the DME, not the O2. 2005-2010 Mazda Speed3/Speed6 with the MZR DISI turbo throws P2243 from the wideband controller module-internal failure. Estimated repair: $40 to $800.