P0387 Crankshaft Position Sensor B Circuit Low Input

P0387 indicates a low input condition in Crankshaft Position Sensor B circuit. The controller is seeing less signal or voltage than expected from the crank B input.

Code details

Generic OBD-II · P0

System: Ignition / Misfire
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not clear P0387 without correcting the low signal source, especially if the engine stalls or will not start.

Symptoms of P0387

Rough idle, shaking, or a stumble under acceleration.
A flashing check-engine light during an active misfire.
Power loss and higher fuel consumption.

Common causes of P0387

Worn spark plugs, ignition coils, or plug wires.
Fuel-delivery problems — injectors, pressure, or a lean/rich mixture.
Vacuum leaks, low compression, or a mechanical fault in that cylinder.

Useful checks before replacing parts

Check crank B supply voltage, ground, signal voltage, waveform amplitude, connector condition, and freeze-frame RPM.
Inspect for short-to-ground faults, open supply circuits, excessive sensor air gap, weak sensor output, damaged wiring, and tone wheel problems.
Measure the signal at both the sensor and controller to separate sensor output problems from harness or module input faults.

Related OBD-II codes

These nearby ignition / misfire entries can help compare wording and pin down the fault area.