P0387

What Does Code P0387 Mean?

The diagnostic trouble code P0387 signifies a detected “Crankshaft Position Sensor B Circuit Low Input” by the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM). The Crankshaft Position (CKP) sensor is a critical component of the engine management system, responsible for monitoring the rotational speed and precise position of the crankshaft. This data is fundamental for the ECM to accurately synchronize ignition timing, fuel injection pulses, and in some applications, valve timing (in conjunction with camshaft position data). Many modern vehicles, particularly those with V-configurations or complex engine designs, may utilize multiple CKP sensors for redundancy or to monitor different aspects of crankshaft rotation. “Sensor B” typically refers to a secondary or redundant CKP sensor, or a specific sensor in a multi-sensor array as defined by the vehicle manufacturer.

A “Circuit Low Input” condition means that the ECM is receiving a voltage signal from CKP Sensor B that is consistently below the specified operational threshold, or is detecting an open circuit where little to no voltage is present. This deviation from expected parameters indicates an electrical fault within the sensor itself, its associated wiring, or potentially the ECM’s ability to interpret the signal. The ECM relies on a clear, consistent signal from the CKP sensor to determine engine RPM and piston position. When this signal is too low or absent, the ECM cannot accurately calculate critical engine operational parameters, leading to compromised engine performance or a no-start condition. The affected subsystem is primarily the engine’s core timing and fuel delivery management.

Common Symptoms

• Engine Cranking But No Start: If the ECM cannot establish crankshaft position, it may prevent spark and fuel delivery.
• Rough Idle or Misfires: Inaccurate timing information can lead to improper spark and fuel delivery.
• Reduced Engine Performance: The ECM may enter a “limp-home” mode or struggle to optimize engine operation, resulting in a noticeable lack of power and responsiveness.
• Engine Stalling: Especially common during deceleration or at inconsistent RPMs if the signal intermittently drops out.
• Intermittent Starting Issues: The engine might start sometimes but not others, suggesting an inconsistent fault.
• Illuminated Check Engine Light (CEL): The primary indicator that a fault has been detected and a DTC stored.
• Erratic Tachometer Readings: Though less common for a secondary “B” sensor, if its signal contributes to RPM calculation, the tachometer might behave erratically.

What Causes the Code P0387?

• Faulty Crankshaft Position Sensor B: Internal electrical failure within the sensor, often due to heat, vibration, or manufacturing defects, resulting in a weak, absent, or incorrect signal output.
• Wiring Harness Issues:
  • Short to Ground: The signal wire or reference voltage wire for CKP Sensor B is inadvertently touching a grounded component, causing the voltage to drop below the acceptable range.
  • Open Circuit: A break or severe corrosion within the signal, reference, or ground wire preventing proper signal transmission.
  • Corrosion or Loose Connections: Degraded terminals or a poor connection at the sensor connector or within the engine harness can increase resistance and lead to a low voltage signal.
• Damaged Reluctor Wheel (Tone Ring): The toothed wheel that the CKP sensor reads from might be physically damaged, bent, or have missing teeth. While often leading to codes like P0335/P0336, a severely damaged wheel could cause a signal so weak it’s interpreted as a “low input.”
• ECM/PCM Failure: While less frequent, an internal fault within the ECM/PCM affecting its sensor input circuit or its ability to supply proper reference voltage or ground to Sensor B can trigger this code.

How to Diagnose and Troubleshoot

Accurate diagnosis of P0387 requires a methodical approach, utilizing an OBD-II scanner and a digital multimeter (DMM). Always consult the vehicle-specific factory service manual for precise wiring diagrams, sensor locations, and voltage specifications.

1. Verify the Code and Check for Related DTCs:
   • Connect an OBD-II scanner and confirm P0387 is active.
   • Check for any accompanying codes (e.g., P0335, P0336 for other CKP issues, or codes related to camshaft position sensors if timing correlation is implied). Freeze frame data can provide valuable information about engine conditions when the fault occurred.
   • Monitor live data, specifically any available CKP Sensor B voltage or RPM signal parameters while cranking or during operation (if the engine runs). Look for erratic, zero, or consistently low voltage readings.
2. Visual Inspection:
   • Locate CKP Sensor B (refer to service manual for exact placement, often near the crankshaft pulley, flywheel/flex plate, or engine block).
   • Inspect the sensor itself for any visible damage, cracks, or signs of impact.
   • Carefully examine the entire wiring harness leading to CKP Sensor B for signs of chafing, cuts, rodent damage, melted insulation, or corrosion. Pay close attention to areas where the harness passes near hot engine components or sharp edges.
   • Disconnect the CKP Sensor B connector. Inspect the connector terminals for bent pins, corrosion, dirt, or a loose fit.
   • If accessible, visually inspect the reluctor wheel (tone ring) for any missing teeth, damage, or excessive debris that could interfere with the sensor’s signal.
3. Digital Multimeter (DMM) Testing (Key On, Engine Off – KOEO):
   • Power Supply and Ground: With the CKP Sensor B connector disconnected (harness side), turn the ignition to the ON position.
     • Identify the reference voltage wire (typically 5V or 12V, consult service manual) and the ground wire.
     • Measure the voltage between the reference voltage pin and a known good chassis ground. Confirm it’s within specification.
     • Measure the resistance between the ground pin and a known good chassis ground. It should be less than 5 Ohms.
   • Signal Wire Continuity and Shorts:
     • Turn the ignition OFF. Disconnect both the CKP Sensor B connector and the ECM/PCM connector (if accessible and necessary to isolate the circuit).
     • Use the DMM to check for continuity on the signal wire between the CKP sensor connector and the ECM connector. Resistance should be very low (typically less than 5 Ohms).
     • Check for shorts to ground: Place one DMM lead on the signal wire terminal at either connector (harness side) and the other lead on a good chassis ground. Resistance should be infinite (OL). A low resistance indicates a short.
     • Check for shorts to voltage: Similar to the above, but test against a known 12V supply wire if the circuit is exposed. Resistance should be infinite (OL).
   • Sensor Resistance (for passive/magnetic sensors): If CKP Sensor B is a passive magnetic reluctance type, disconnect it and measure its internal resistance across its terminals. Compare to factory specifications. A resistance outside the specified range indicates an internal sensor fault. (Note: Hall effect sensors are active sensors and cannot be tested for resistance in this manner).
4. Oscilloscope Testing (Advanced):
   • For Hall effect or magnetic reluctance sensors, an oscilloscope can provide a definitive test of the sensor’s output signal. Back-probe the sensor connector while cranking the engine. A Hall effect sensor should produce a clean, square-wave pattern, while a magnetic reluctance sensor will produce an AC sine wave. A “low input” might manifest as a weak, distorted, or completely absent waveform.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are commonly recommended:

• Replace Faulty Crankshaft Position Sensor B: If all wiring and power/ground checks confirm integrity, and sensor-specific tests (resistance or oscilloscope) indicate failure, replacement of the CKP Sensor B is the most frequent solution. Always use a high-quality OEM or equivalent replacement part to ensure proper function and longevity.
• Repair or Replace Wiring Harness: If the diagnosis points to a short to ground, open circuit, or damaged wiring/connector, the affected section of the harness must be repaired. Utilize proper automotive wiring techniques, including soldering, heat-shrink tubing, and weather-sealed connectors to ensure a durable and reliable repair. In cases of extensive damage, replacing the entire engine wiring sub-harness may be necessary.
• Clean or Repair Connector Terminals: Corroded, bent, or spread pins within the sensor or ECM connector should be carefully cleaned with electrical contact cleaner or replaced/repaired using terminal repair kits. Ensure a tight, secure connection.
• Inspect and Repair Reluctor Wheel: If inspection reveals a damaged reluctor wheel (missing teeth, bent, or debris accumulation), it must be repaired or replaced. This often requires significant mechanical work, such as removal of the harmonic balancer or flywheel/flex plate, depending on its location.
• ECM/PCM Replacement/Reprogramming: Only consider ECM/PCM replacement as a last resort, after meticulously ruling out all other possibilities and confirming internal ECM failure. This is an expensive repair and often requires reprogramming the new module to the vehicle.

Important Mechanics’ Tips:

• Always disconnect the vehicle’s battery before performing any electrical repairs or component replacements.
• When replacing a CKP sensor, ensure the mounting surface is clean and free of debris.
• After completing repairs, clear all stored DTCs using an OBD-II scanner. Perform a comprehensive test drive under various engine loads and speeds to confirm the repair and ensure the P0387 code does not return, and all readiness monitors reset.
• Pay close attention to sensor routing; ensure wiring is secured away from hot exhaust components, moving parts, and sharp edges to prevent future damage.