P0387
UniversalCrankshaft Position Sensor B Circuit Low Input
The engine computer detects a voltage or signal level that is too low in the Crankshaft Position Sensor B circuit. The crankshaft position sensor helps the computer track crankshaft movement and manage engine timing. A low signal can interfere with starting or engine operation, but the code does not by itself prove the sensor needs replacement. Sensor B identification and circuit design vary by vehicle.
⚠️ Before You Begin (Safety First)
If the engine stalls, is difficult to keep running, or loses power, avoid driving in traffic; arrange a safe stop and assistance. Do not continue driving if stalling makes the vehicle difficult to control.
Common Symptoms
- The engine may crank but fail to start if the computer cannot obtain a usable crankshaft signal.
- The engine may start inconsistently or run roughly if the low signal disrupts timing information.
- The engine may stall or lose power if the signal drops out while running.
- The check-engine light may be on, with or without noticeable drivability symptoms.
Common Causes
- A faulty Crankshaft Position Sensor B may produce an abnormally low signal.
- Damaged, corroded, loose, or contaminated wiring or connector contacts may reduce or interrupt the circuit signal.
- A circuit short to ground or a problem with circuit power or ground, where applicable to the vehicle design, may cause a low input.
- A mechanical or installation issue affecting the sensor's ability to read crankshaft movement may contribute to an abnormal signal.
- A control-module or related circuit fault is possible, but should be considered only after the sensor and wiring have been tested.
Diagnostic Steps
- Record the code and check for other stored codes before clearing anything; related codes can help show whether the concern affects a broader circuit or engine signal.
- With the engine off, inspect accessible Sensor B wiring and connectors for looseness, corrosion, damaged insulation, or signs of rubbing. Do not pull on wires or disconnect connectors while the engine is running.
- Check the vehicle-specific service information to identify Sensor B and learn what inspection or testing is appropriate for that engine; its location and circuit design vary.
- If the code returns or symptoms occur, arrange circuit testing rather than replacing the sensor based on the code alone. Seek help promptly if the engine stalls or will not start.
When to Call a Professional
Use vehicle-specific service information to identify Sensor B and its circuit. A technician can inspect related codes and live data, check the connector and wiring, test circuit power and ground where applicable, and compare the sensor signal with the expected pattern using suitable diagnostic equipment. Check the circuit before replacing the sensor.
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Commonly Asked Questions
Does P0387 prove that Crankshaft Position Sensor B is faulty?
No. It identifies a low input in the Sensor B circuit. The sensor, wiring, connector, circuit power or ground where applicable, and related mechanical conditions need to be considered.
What does Sensor B mean, and where is it located?
Sensor B distinguishes this crankshaft-position signal from another sensor or circuit designation used in some vehicle designs. Its exact identity and location depend on the model, year, and engine.
Can I drive with P0387?
If the engine runs normally, the code alone does not establish how the vehicle will behave. However, because signal problems can cause stalling or starting trouble, stop driving if the engine stalls, loses power, or becomes difficult to control.
Should I replace the sensor first?
Not without testing. A damaged connector or wiring fault can produce a low input too, so checking the circuit and signal helps avoid replacing a good sensor.
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