P0387

P0387 means the CKP-B signal is reading LOW — either signal voltage is below the controller's lower threshold (typically under 0.3 V on a Hall-effect output that should swing 0-5 V, or under 0.3 V AC peak on a VR sensor at cranking when it should produce 0.5-1.5 V AC), or the signal-return circuit is shorted to ground. Cheapest-first ladder: (1) key-on, backprobe the CKP-B signal wire at the sensor connector and read voltage with the DMM — a Hall sensor sits high (5 V) or low (0 V) at rest depending on the reluctor tooth position, and a wire stuck at 0 V key-on regardless of crank position confirms a short-to-ground in the signal lead or a dead sensor pulling its output low. (2) Disconnect the sensor and re-measure at the harness side — if the harness pin now reads the expected 5 V supply, the sensor was internally shorted and is the fault; if it still reads 0 V, the wiring is shorted or the PCM-side supply is dead. (3) Crank-test with a scope and verify pulse amplitude: a healthy Hall CKP-B shows clean 0-5 V transitions at 300+ pulses/sec at 200 rpm cranking; a VR sensor shows 0.5-1.5 V AC at cranking and grows to 5-15 V peak-to-peak by 2,500 rpm — amplitude under 0.3 V at cranking on a VR sensor confirms a weak/dead coil or excessive air gap. (4) Inspect the connector pins for corrosion and verify the shield drain wire is grounded on shielded-cable applications. Expensive-misdiagnosis caveat: before replacing the CKP-B, check the air gap and reluctor — an air gap above spec (over 1.5 mm on most applications) reads as low signal amplitude on a VR sensor and trips P0387; the sensor and harness are fine, but the mounting boss has been damaged or the shim is missing.

Vehicle-specific patterns: 2002-2014 Subaru EJ-series throws P0387 from a chronic mounting-boss crack on the secondary sensor that lets the sensor sit too far from the reluctor, dropping VR amplitude below threshold. 2004-2013 Subaru turbo (WRX/STI) throws P0387 acutely from the same root cause exacerbated by heat. 2003-2010 Ford 6.0L Powerstroke throws P0387 from a chafed signal wire near the front cover that shorts intermittently to the cover itself. 2001-2010 GM Duramax LB7-LMM throws P0387 from corroded sensor-connector pins where the harness routes near the steering knuckle and sees salt spray. 2004-2011 Mazda RX-8 secondary back-up sensor throws P0387 from oil contamination at the tip. Estimated repair: $130 to $550.