P0387 Code: Crankshaft Position Sensor B Circuit Low Input — Symptoms, Causes, and Repair

Quick answer

• What it means: The Powertrain Control Module (PCM) is detecting a lower-than-expected electrical signal voltage from the Crankshaft Position Sensor (CKP) "B" circuit.
• Severity: High (Do not drive). This code can cause your engine to stall unexpectedly, run terribly, or fail to start altogether. It is highly recommended to get the vehicle towed to a shop.
• Most common cause: A failing crankshaft position sensor or damaged wiring in the sensor harness.

What the P0387 code means

The official OBD-II definition for P0387 is Crankshaft Position Sensor "B" Circuit Low Input.

To understand this, you need to know what the Crankshaft Position Sensor (CKP) does. This critical sensor reads the rotational speed (RPM) and exact position of the engine's crankshaft by monitoring a toothed reluctor ring (or tone ring) attached to the crankshaft. The engine computer (PCM) uses this precise data to determine exactly when to fire the spark plugs and pulse the fuel injectors.

Many modern vehicles use two crankshaft position sensors (Sensor A and Sensor B) for redundancy and increased accuracy, particularly in complex variable valve timing systems. When the PCM sets code P0387, it means the electrical voltage signal coming from Sensor B has dropped below the manufacturer's specified minimum threshold. In plain English: the engine's computer has lost the signal from one of its most important timing sensors, and it no longer knows exactly where the engine's internal components are in their rotation.

Common symptoms

Because the crankshaft position sensor dictates critical fuel and spark timing, a low input circuit will drastically affect engine performance. You will likely experience:

• Illuminated Check Engine Light: The first and most obvious sign.
• Engine stalling: The engine may suddenly shut off while driving or idling when the signal drops out completely.
• Hard starting or no-start condition: The engine may crank normally but refuse to fire up because the PCM is withholding spark and fuel.
• Rough idle and misfires: The engine may sputter, shake, or run erratically due to improper ignition timing.
• Reduced engine power: The vehicle may enter "limp mode" to protect internal components.
• Poor fuel economy: Inaccurate timing leads to inefficient combustion.

Common causes

Ranked from most to least likely, the culprits behind a P0387 code include:

1. Defective Crankshaft Position Sensor B: Internal failure of the sensor due to extreme engine heat and vibration is the most common cause.
2. Damaged wiring or loose connectors: Corroded, melted, or frayed wiring in the sensor's harness can cause a voltage drop, triggering the low input code.
3. Oil or debris contamination: An oil leak or metal shavings covering the magnetic tip of the sensor can weaken its ability to read the reluctor ring.
4. Damaged reluctor ring: If the teeth on the reluctor (tone) ring break, bend, or become heavily rusted, the sensor will not generate a proper signal.
5. Incorrect sensor air gap: If the sensor was recently replaced but not shimmed properly, it may sit too far from the reluctor ring, creating a weak signal.
6. Failing PCM (Rare): The engine computer itself could have a burned-out driver or internal fault, though this is the least likely scenario.

How a technician diagnoses it

An ASE-certified technician will follow a systematic process to pinpoint the exact cause of the P0387 code, ensuring no good parts are replaced unnecessarily.

1. Scan and retrieve freeze frame data: The tech will use a professional-grade scan tool to verify the P0387 code, check for related codes, and review the freeze-frame data to see the exact engine conditions (RPM, temperature, load) when the code triggered.
2. Visual inspection: A thorough check of the CKP Sensor B, its wiring harness, and connector. The tech is looking for melted wires (from exhaust heat), oil saturation, corrosion in the connector pins, or signs of rodent damage.
3. Sensor resistance and voltage testing: Using a digital multimeter (DMM), the technician will check the sensor's internal resistance (if it's a VR/magnetic type) or test the 5-volt reference and ground circuits (if it's a Hall-effect type). A healthy 5V reference circuit should read right at 5.0 volts. If it's reading 1.5 volts, there is a wiring issue.
4. Oscilloscope analysis: The most definitive test. A master tech will hook up an oscilloscope to the sensor signal wire to watch the waveform in real-time while cranking or running the engine. A Hall-effect sensor should produce a crisp 0-to-5V square wave. If the wave is highly distorted, weak in amplitude, or dropping out entirely, the sensor or reluctor ring is faulty.
5. Reluctor ring inspection: If the waveform shows specific missing pulses, the tech will remove the sensor and inspect the reluctor ring through the mounting hole (rotating the engine by hand) to check for missing or damaged teeth.

Typical repair cost

The cost to fix a P0387 code will vary depending on the root cause and the make and model of your vehicle. Access to the sensor (some are easily reachable on the front of the block; others are buried behind the transmission) heavily dictates labor time.

• Crankshaft Position Sensor replacement: $150 – $350 (Parts: $50–$100, Labor: $100–$250).
• Wiring repair: $100 – $200 (Labor to locate and repair broken or shorted wires).
• Reluctor ring replacement: $500 – $1,200+ (Requires significant engine or transmission teardown depending on the vehicle).

Can I keep driving with this code?

No. It is not safe to keep driving with a P0387 code.

A failing crankshaft position sensor is highly unpredictable. While you might currently just have a Check Engine Light and a slightly rough idle, the sensor signal could drop out completely without warning. If this happens while you are driving at highway speeds or turning across traffic, your engine will stall immediately. You will lose power steering and power brakes, putting you at serious risk of an accident. If your car is running, drive it straight to a repair facility. If it's stalling or running terribly, get it towed.

Related codes

You will often see P0387 accompanied by other DTCs. Some common related codes include:

• P0385: Crankshaft Position Sensor "B" Circuit Malfunction
• P0386: Crankshaft Position Sensor "B" Circuit Range/Performance
• P0388: Crankshaft Position Sensor "B" Circuit High Input
• P0389: Crankshaft Position Sensor "B" Circuit Intermittent
• P0335: Crankshaft Position Sensor "A" Circuit Malfunction (indicates the primary sensor is also having issues)
• P0340: Camshaft Position Sensor Circuit Malfunction (often accompanies CKP codes since they work in tandem to determine timing)

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