P0388

P0388 means the CKP-B signal is reading HIGH — signal voltage is above the controller's upper threshold (typically above 4.8 V on a Hall-effect output, or above 15 V peak on a VR sensor that should max at 5-15 V peak-to-peak at 2,500 rpm), or the signal wire is shorted to the 5 V reference or to battery voltage. Cheapest-first ladder: (1) key-on, backprobe the CKP-B signal wire and read voltage with the DMM — a Hall sensor stuck at 5 V regardless of crank position confirms either an internal sensor failure or a short to the 5 V reference. (2) Disconnect the sensor connector and re-read at the harness — if the harness pin now reads 5 V with the sensor disconnected, the wire is shorted to the reference inside the harness; if it reads the expected open-circuit value, the sensor is internally failed. (3) Scope the signal at cranking — a healthy Hall CKP-B swings 0-5 V cleanly, and a VR sensor produces 0.5-1.5 V AC at cranking, rising to 5-15 V peak-to-peak by 2,500 rpm. A VR output above 18 V peak indicates a shorted-turn condition or a short to a higher-voltage line. (4) Check for water intrusion at the sensor connector — moisture between the signal and 12 V pins bridges the two and reports as 'high circuit voltage.' Expensive-misdiagnosis caveat: before replacing the CKP-B sensor, verify the 5 V reference at the PCM-side connector is the correct 5.0 V (+/- 0.1 V) — a failing PCM reference regulator that's outputting 7-9 V will cause every Hall-style sensor on that reference bus to read as 'high circuit'; replacing one sensor won't fix the problem and the next sensor will set the same code.

Vehicle-specific patterns: 2002-2014 Subaru EJ-series throws P0388 from a known harness rub-through near the alternator bracket that shorts the secondary CKP signal wire to the 12 V alternator output line. 2004-2011 Mazda RX-8 throws P0388 from internal sensor failure on the secondary rotor-position back-up sensor — the tip soaks oil over time and the internal Hall element shorts. 2007-2014 RAM Cummins 6.7L throws P0388 from water intrusion at the CKP connector behind the starter, where the connector boot degrades and lets moisture in. 2008-2013 BMW N57 inline-6 diesel throws P0388 from a known sensor failure where the Bosch unit's internal output transistor shorts to the supply rail. Estimated repair: $140 to $600.