P0387 indicates the powertrain control module (PCM) has detected an abnormally low voltage signal from the Crankshaft Position (CKP) Sensor 'B' circuit. Many engines use two CKP sensors—one as the primary timing reference and a second for misfire detection, cylinder identification, or backup—and this code points specifically to the secondary sensor reading below the expected threshold.
The sensor's internal Hall-effect or magnetic element degrades, shorts internally, or loses sensitivity, producing a low or absent signal. This is the single most common cause of P0387.
Chafed, heat-soaked, or oil-contaminated wiring and corroded, loose, or backed-out pins in the sensor connector are frequent, especially on harnesses routed near the lower block, bellhousing, or oil pan.
Rust, debris, a missing tooth, or excessive runout on the crankshaft tone wheel that the sensor reads can drop the signal amplitude below the PCM's threshold.
If the sensor was installed loosely, its bracket is cracked, or the mounting bolt is over- or under-torqued, the gap between the sensor tip and the tone wheel can be too large, weakening the induced signal.
A corroded engine ground strap or a short-to-ground in the sensor's reference-voltage wiring can pull the CKP B signal low even when the sensor is good.
The powertrain control module itself has a failed input stage for the CKP B circuit. Genuine PCM failures are uncommon for this code and should be considered only after the sensor, harness, and tone wheel have all been ruled out.
Short trips may be possible, but P0387 can cause stalling, a no-start condition, or limp-mode operation. It should be diagnosed promptly to avoid being stranded.
It is typically mounted on the engine block—often near the crankshaft snout, behind the harmonic balancer, at the transmission bellhousing, or on the oil pan rail. Exact placement varies by engine family, so a model-specific service manual is the most reliable reference.
Usually no. P0387 is far more often caused by the sensor itself, its connector, the wiring, or the tone wheel. Replacing the PCM is expensive and rarely the actual fix; the module should be suspected only after the sensor, the entire harness, and the tone wheel have all been thoroughly tested and ruled out.
No. P0335 and P0336 refer to CKP Sensor A, the primary timing reference. P0387 specifically refers to CKP Sensor B, the secondary sensor used on engines with dual crankshaft-position inputs.