P0238

P0238 is the boost sensor A circuit high fault, set when the sensor signal voltage exceeds the calibrated upper threshold, typically above 4.8 volts on a 0 to 5 volt sensor. Cheapest first: scan the sensor at key-on-engine-off; a reading pinned at 4.9 to 5.0 volts or showing pressure well above atmospheric (over 18 psi absolute at sea level) indicates a circuit or sensor fault. Disconnect the sensor and verify the signal voltage drops to 0 volts at the ECM connector; if it stays high, you have a short between the signal wire and the 5-volt reference. If signal drops correctly with the sensor unplugged, the sensor itself is faulty. Check the ground pin: an open ground at the sensor will float the signal voltage to the rail. Inspect the connector for water intrusion and corrosion, especially on under-hood sensors mounted near the front of the engine bay where water spray is common. Caveat: a mechanical overboost condition where actual boost truly does exceed the sensor's calibrated range can set P0238 with a perfectly good sensor, so verify with a mechanical boost gauge if no wiring fault is found.

Vehicle-specific patterns: Ford 6.0L and 6.4L Power Stroke (2003-2010) sees P0238 from boost sensor connector corrosion. Cummins ISB 6.7L (2007-2018 Ram HD) sets P0238 from boost sensor harness chafe at the turbo support bracket. VW/Audi 2.0T TFSI (2008-2017) sees P0238 from manifold pressure sensor failure and connector wear. Mercedes OM651 2.1 CDI (2009-2016) sets P0238 from boost sensor failure as a maintenance item. Estimated repair: $40 to $350.