P0387

What it means

P0387 means the engine control unit has detected a low voltage signal in the crankshaft position sensor 'B' circuit. The ECU saw the signal voltage fall below the expected range, which points to a short to ground, an open circuit or a failing sensor. Which sensor counts as 'B' and how it is wired depends on the make, model and engine.

Symptoms

• Engine cranks but is hard to start or will not start
• Engine stalls unexpectedly or cuts out while driving
• Misfires, hesitation or rough running
• Engine management light on
• Tachometer flickering or dropping to zero
• Reduced power or the car going into limp mode

Possible causes

• Faulty crankshaft position sensor 'B'likely
• Damaged, chafed or shorted wiring to the sensor, including a short to ground or an open circuit in the signal or supply wirelikely
• Corroded, loose or contaminated sensor connector or terminalslikely
• Loss of sensor reference supply voltage or a poor ground from the ECU or a shared circuitpossible
• Damaged or contaminated reluctor wheel or trigger ring, or incorrect sensor air gap or mountingpossible
• Fault inside the engine control unitrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data (engine speed, load, coolant temperature) to see when the fault set.
2. Check for related codes such as other crankshaft or camshaft sensor faults, which can point to a shared supply or ground fault.
3. Inspect the sensor, its wiring and the connector for chafing, oil contamination, heat damage, corrosion and a secure fit.
4. Watch live engine speed data while cranking and running, and check whether the signal drops out when the wiring is wiggled.
5. Using the manufacturer's wiring diagram for the model, test the sensor supply voltage, signal wire and ground for continuity and shorts to ground.
6. Measure the sensor's resistance or output signal against the manufacturer's specification, as the method depends on the sensor type.
7. Inspect the reluctor wheel for damage or debris, and check the sensor mounting and air gap where applicable.
8. If the wiring and sensor test fine, check the ECU power and ground supplies before suspecting the control unit.

How to fix

• Replace the crankshaft position sensor 'B' if it fails testing, using a correct specification part.
• Repair or replace damaged wiring and fix any short to ground or open circuit.
• Clean, repair or replace corroded or loose connectors and terminals.
• Restore the correct supply voltage or ground connection to the sensor circuit.
• Replace or repair a damaged reluctor wheel and correct the sensor fitment or air gap.
• Repair or replace the engine control unit only once all other causes have been ruled out and the fault is confirmed.

Can I keep driving?

Nearby codes

• P0386GenericCrankshaft Position Sensor B Circuit Range/Performance
• P0388GenericCrankshaft Position Sensor B Circuit High