Crankshaft Position Sensor B Circuit Low Input
The PCM detected a low input voltage signal from the secondary crankshaft position (CKP) sensor circuit, which monitors engine speed and crankshaft position. This sensor is critical for engine timing and fuel injection; without proper signal, the engine cannot run reliably.
What You Might Notice
- Engine will not start or has severe starting difficulty
- Tachometer not reading or displaying erratically
- Engine stalling during operation
- Rough idle and poor engine performance
- Check Engine light illuminated
Most Common Causes
- 1
Faulty Secondary CKP Sensor
The CKP sensor B may be failing or producing a weak signal due to internal damage or wear. This is the most common cause of low input voltage from the sensor circuit.
- 2
Wiring or Connector Issues
Corroded, loose, or damaged wiring in the CKP sensor B circuit can cause poor signal transmission. Check connectors for corrosion, loose pins, or broken wires between sensor and PCM.
- 3
Low Battery Voltage
Insufficient battery voltage can cause the sensor to produce a weak signal that the PCM interprets as low input. Verify battery is properly charged and connections are clean.
- 4
PCM Electrical Fault
Rarely, a faulty PCM input circuit or module defect can cause false low voltage detection. This should be diagnosed only after sensor and wiring are confirmed good.
How to Diagnose It
- 1
Check Faulty Secondary CKP Sensor — The CKP sensor B may be failing or producing a weak signal due to internal damage or wear. This is the most common cause of low input voltage from the sensor circuit.
- 2
Check Wiring or Connector Issues — Corroded, loose, or damaged wiring in the CKP sensor B circuit can cause poor signal transmission. Check connectors for corrosion, loose pins, or broken wires between sensor and PCM.
- 3
Check Low Battery Voltage — Insufficient battery voltage can cause the sensor to produce a weak signal that the PCM interprets as low input. Verify battery is properly charged and connections are clean.
- 4
Check PCM Electrical Fault — Rarely, a faulty PCM input circuit or module defect can cause false low voltage detection. This should be diagnosed only after sensor and wiring are confirmed good.
How to Fix It
- 1
Replace Secondary Crankshaft Position Sensor
Remove and replace the CKP sensor B with a new OEM or quality replacement. Ensure proper installation and torque specifications per manufacturer guidelines.
- 2
Inspect and Repair Wiring
Check the entire CKP sensor B wiring harness for corrosion, damage, or loose connections. Repair or replace damaged wires and clean corroded connectors with electrical contact cleaner.
- 3
Test and Charge Battery
Perform a battery voltage test to ensure it meets manufacturer specifications (typically 12.6V at rest). Charge or replace the battery if voltage is low, and clean all battery terminals.
- 4
Clear Code and Road Test
After repairs, use a diagnostic scanner to clear the P0387 code and verify no other related codes are present. Road test the vehicle to confirm normal operation and that the code does not return.
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