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P0387 OBD-II Trouble Code: Crankshaft Position Sensor B Circuit Low Input

check engine light

P0387 code definition

When the PCM (powertrain control module) detects a signal from crankshaft position sensor B that is outside the normal variation set by the manufacturer, the P0387 code is set and the Check Engine Light is illuminated.

What the P0387 code means

Under normal circumstances, the crankshaft position sensor records and transmits the speed at which the crankshaft is spinning and its current position to the PCM. This data allows the PCM to determine such functions as ignition timing and fuel delivery. When the circuit input for this sensor is too low, erratic, or otherwise outside normal bounds by more than 10%, the P0387 code is stored because the incorrect voltage input can negatively affect the engine’s function.

What causes the P0387 code?

The P0387 code has a number of causes, including:

  • A short or opening in the crank shaft position sensor’s wiring harness or circuit
  • A defective crankshaft position sensor
  • A poor electrical connection to the crankshaft position sensor
  • A damaged or defective starter motor or starting system circuit
  • Damage to the signal plate
  • An undercharged or dead battery

What are the symptoms of the P0387 code?

In all cases, when the P0387 code is stored the Check Engine light will illuminate. You may also notice a couple of other symptoms, as well, including:

  • Engine misfires
  • Choppy or uneven acceleration
  • Lack or loss of power
  • Engine stall
  • Rough idling
  • Decreased fuel efficiency

How does a mechanic diagnose the P0387 code?

Using an OBD-II scanner, the mechanic will see that the P0387 code has been stored, as well as any other codes that may currently be stored by the PCM. Before beginning diagnosis on the P0387 code issue, they will first address any codes that were stored before this code, as one of those could be the key to the problem. After diagnosing and repairing each subsequent code, the mechanic will clear all codes and retest the system.

If the P0387 code is not resolved after dealing with previous codes, the mechanic will perform a visual inspection of the crankshaft position sensor, the crankshaft gear, and all relevant components, wiring, and connectors. They will then work their way through the possible causes, clearing the codes and retesting the system after each, until the problem is resolved.

Common mistakes when diagnosing the P0387 code

Failure to thoroughly inspect the wiring and connectors to and from the crankshaft position sensor often leads to undiagnosed intermittent trouble. In this case, there may be a loose connection or a wire that functions intermittently, causing the code to be set when the connection is lost. A complete inspection of these components can avoid this problem.

How serious is the P0387 code?

If the PCM is unable to accurately predict proper ignition timing and fuel delivery, the vehicle will have noticeable problems with drivability and performance. These problems can lead to further engine damage, and driving with unreliable acceleration and other problems can be dangerous, as well.

What repairs can fix the P0387 code?

There are a number of problem(s) that could be causing a P0387 code to be stored. Depending on the root of the code, one or more of these repairs may resolve it:

Additional comments for consideration regarding the P0387 code

Even if your vehicle starts and stops with no issues and the symptoms associated with the P0387 code are not significant, this code should be dealt with immediately. The conditions causing it to occur can get worse over time, and the illuminated Check Engine light will prevent the vehicle from passing an OBD-II emissions test.

Need help with a P0387 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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