P0387
Crankshaft Position Sensor B Circuit Low Input
The appearance of the standard OBD2 trouble fault code P0387 is an indicator that your vehicle ECU triggered a threshold alert for "Crankshaft Position Sensor B Circuit Low Input". Understanding the root component breakdown helps avoid expensive diagnostic fees.
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.
Related Fault Codes
Frequently Asked Questions
What does OBD2 code P0387 mean?
OBD2 code P0387 refers to Crankshaft Position Sensor B Circuit Low Input. The appearance of the standard OBD2 trouble fault code P0387 is an indicator that your vehicle ECU triggered a threshold alert for "Crankshaft Position Sensor B Circuit Low Input". Understanding the root component breakdown helps avoid expensive diagnostic fees.
What are the most common symptoms of DTC P0387?
Drivers experiencing code P0387 typically report: Engine cranks but will not start.
Is it safe to drive with trouble code P0387 active?
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.
How much does it cost to fix code P0387?
The estimated repair cost for P0387 ranges from $100 to $400 USD. Sensor replacement: 150 - 350
Sensor replacement: 150 - 350
Need Professional Diagnostic Assistance?
Always verify sensor signals and wiring integrity with an OBD2 scanner before purchasing replacement parts.
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