P0387
Crankshaft Position Sensor B Circuit Low Input
Encountering the engine check light code P0387 signifies an explicit mechanical or electrical operational breakdown categorized as "Crankshaft Position Sensor B Circuit Low Input". Professional scanner tools usually flag this subsystem loop early on.
Driver's Summary
The diagnostic trouble code P0387 indicates an active fault in the crankshaft position sensor b circuit low input circuit or component. The most common signs are engine cranks but will not start. This condition is classified as high severity. Prompt diagnosis is essential to prevent cascading damage to related components.
Symptoms
Engine cranks but will not start
Common Causes
- Short to ground in sensor B circuit
- Dead crankshaft sensor B
- Low battery voltage
- PCM sensor ground failure
How to Fix
- 1 Trace and repair short to ground
- 2 Replace crankshaft position sensor B
- 3 Test charging system
- 4 Test PCM ground circuit
Technical Explanation
P0387 is stored after the control module confirms the fault over multiple ignition cycles, ruling out transient electrical noise as the cause. For injector-specific codes, the ECM monitors the injector control circuit voltage drop during each pulse; a shorted or open injector presents a characteristic resistance signature that differs measurably from a healthy unit. Once confirmed, the code is stored as a permanent DTC and the MIL is activated. The freeze frame snapshot — recording RPM, load, coolant temperature, and fuel trim at fault detection — is also saved and is critical for accurate diagnosis.
Is It Safe to Drive?
This fault carries real mechanical risk. The root causes — including short to ground in sensor b circuit — can trigger a chain reaction of component failures if the vehicle continues to be driven. Have it towed or drive directly to a shop without delay.
Mechanic's Pro Tip
Module replacement should always be the last resort for P0387 after exhaustively verifying all power supply circuits, ground connections, and communication bus wiring. Use a wiring diagram to locate all fuses, relays, and ground points for the affected module, and measure voltage drop on each ground with the circuit loaded. A module "failure" is frequently a corroded ground eyelet or a weak battery causing brownout conditions — fix these first and you'll save hundreds of dollars on an unnecessary module replacement.
Sensor replacement: 150 - 350