Engine

P0387 — Crankshaft Position Sensor B Circuit Low Input

Severity: high Drive with care, to a shop

What P0387 means

The engine control module uses the crankshaft position sensor signal to measure engine speed and shaft position, which it needs to time spark and fuel injection. P0387 sets when the voltage in the sensor B circuit stays below the expected range. Typical causes are an open or grounded wire, a failed sensor, or, less often, a fault in the ECM input.

Symptoms

  • Check Engine light on
  • Engine cranks but does not start, or starts hard
  • Tachometer does not move while cranking
  • Hesitation, stumbling or loss of power
  • Rough idle or sudden stalling
  • Worse fuel economy

Possible causes

  1. Failed crankshaft position sensor B
  2. Damaged wiring, corroded terminals or a poor connector
  3. Signal wire shorted to ground
  4. Open circuit in the sensor power or reference wire
  5. Damaged or dirty reluctor wheel (tone ring)
  6. Fault in the ECM input circuit

How to diagnose

  1. Scan all codes and freeze frame data, and look for camshaft sensor or misfire codes too.
  2. Inspect the sensor, connector and harness for chafing, oil, heat damage and corroded pins.
  3. Check whether the tachometer and scan tool show RPM while cranking.
  4. Use a multimeter to check sensor power and ground at the connector, following the wiring diagram for the vehicle.
  5. Test the signal wire for opens and shorts to ground with the ECM connector unplugged.
  6. Test the resistance of an inductive sensor, or scope the signal if it is a Hall-effect type.
  7. Inspect the reluctor wheel for damaged teeth or debris and verify the sensor air gap.
  8. If wiring and sensor are good, verify ECM power and ground, then suspect the ECM.

Common fixes

  • Replace crankshaft position sensor B
  • Repair the wiring harness, replace a damaged connector or clean the terminals
  • Repair or replace the reluctor wheel and set the correct gap
  • Reprogram or replace the ECM once it is confirmed faulty

Questions and answers

What does P0387 mean?

The ECM detected low voltage in the crankshaft position sensor B circuit. The sensor or its wiring is the most common cause.

Can I drive with P0387?

It is not recommended, since the engine may stall suddenly or fail to start. If you must, keep it to a short trip to a repair shop.

What usually causes P0387?

Most often a failed sensor or a damaged harness or connector. ECM faults are rare and should be checked last.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.