P0239

P0239 means the secondary boost pressure sensor (Sensor B — used on twin-turbo applications, sequential-turbo systems, or engines with both a pre-throttle and post-throttle MAP) has a general circuit malfunction. Sensor B is typically located on the opposite bank from Sensor A on a parallel twin-turbo (BMW N54/N63), or in the inter-turbo plumbing on a sequential setup (older BMW M57 diesel, Mazda CX-7 2.3L DISI). The cheapest-first ladder: identify which sensor is Sensor B by the FSM wiring diagram (don't guess — manufacturers number these inconsistently), backprobe the signal wire, and confirm the voltage. A healthy boost sensor at key-on-engine-off should read 1.5-2.0V at atmospheric pressure; under wide-open-throttle boost of 15-20 psi, voltage should climb to roughly 3.5-4.2V on a 0-5V sensor. Check the connector for oil contamination and the wiring harness for chafe points (Sensor B's harness often runs along the firewall or under the intake plenum and gets pinched during prior turbo or manifold work). Verify the 5V reference and ground are within spec. Caveat: on twin-turbo applications, the expensive misdiagnosis is replacing both turbos when a single $50 sensor was the actual fault — always confirm with a smoke test and a hands-on actuator check before pulling turbos.

Vehicle-specific patterns: 2007-2010 BMW N54 (335i, 535i, X6 35i) throws P0239 from oil-fouled Sensor B on the rear turbo, often appearing alongside HPFP and wastegate codes — the rear sensor lives in a heat-soaked location and the connector pins corrode. 2008-2013 BMW N63 4.4L twin-turbo V8 (550i, 750i, X5 50i) frequently throws P0239 from the documented charge-pipe failure that exposes the rear-bank sensor to coolant/oil mist; BMW Customer Care Package covers many VINs. 2007-2012 Mazda CX-7 2.3L DISI turbo throws P0239 from sequential boost control sensor failures tied to chronic turbo oil-feed restriction. 2016-2019 Ford F-150 3.5L EcoBoost (second-gen) occasionally throws P0239 when the secondary MAP in the charge-air system fails or develops a high-resistance connection. Estimated repair: $50 to $600.