P0387 Crankshaft Position Sensor "B" Circuit Low Input

The P0387 diagnostic trouble code (DTC) means that the engine control module (ECM/PCM) has detected a lower-than-expected electrical signal from the Crankshaft Position Sensor “B” circuit.

The crankshaft position sensor is responsible for providing the engine computer with information about crankshaft position and rotational speed. This information is fundamental to engine synchronization, fuel injection, ignition timing, RPM calculation, and, depending on the vehicle, variable valve timing and misfire detection.

The word “Low” in P0387 refers primarily to the electrical signal seen by the ECM, not necessarily to low engine RPM, low crankshaft position, or low physical engine speed.

A low-input code also does not automatically prove that the crankshaft position sensor has failed. A short to ground, damaged wiring, poor connection, incorrect power supply, sensor ground problem, damaged trigger wheel, or another circuit fault can produce the same code.

What Does Crankshaft Position Sensor “B” Do?

The crankshaft position sensor monitors the rotation of the crankshaft using a toothed, slotted, magnetic, or otherwise patterned trigger mechanism.

As the crankshaft rotates, the sensor generates a signal that allows the ECM to determine:

• Crankshaft position

• Engine speed

• Crankshaft acceleration and deceleration

• Engine synchronization

• Ignition timing

• Fuel injection timing

• Misfire information

On engines with variable valve timing, the crankshaft signal is also used together with camshaft position information to determine whether the camshafts are actually positioned where the ECM expects them to be.

The “B” designation is manufacturer-dependent. It does not universally identify the same physical sensor location on every vehicle.

What Does “Low Input” Mean?

In P0387, “low input” means the ECM is detecting a crankshaft position sensor circuit signal below the expected electrical level.

Depending on the sensor design, this can happen because:

• The signal wire is shorted to ground

• The sensor output is weak

• The sensor has an internal fault

• The sensor has a poor ground

• The sensor has an incorrect power supply

• A connector terminal has excessive resistance

• Wiring is damaged

• The trigger mechanism is damaged

• The sensor is incorrectly positioned

• Another electrical fault is pulling the circuit voltage down

It does not mean that the engine is simply turning at a low RPM.

For example, an engine could be cranking at a normal speed while the ECM sees an abnormally low sensor signal.

Hall-Effect and Variable-Reluctance Sensors

Crankshaft position sensors use different technologies, so the electrical behavior of the circuit depends on the vehicle.

A Hall-effect sensor generally receives an external power supply and produces a switching signal.

A variable-reluctance (VR) sensor generates an alternating electrical signal as the trigger wheel passes the sensor.

Because of these differences, there is no single voltage or resistance value that can be used to diagnose every P0387 fault.

The correct specifications must come from the vehicle manufacturer's service information.

This is particularly important when testing resistance. A resistance value that looks reasonable on one sensor design may be completely inappropriate for another.

Symptoms of P0387

The symptoms depend on how low the signal is and whether the ECM can still interpret it reliably.

Possible symptoms include:

• Check Engine Light

• Difficult starting

• Extended cranking

• Intermittent no-start

• Engine stalling

• Rough running

• Hesitation

• Poor acceleration

• Reduced engine performance

• Unstable RPM readings

• Engine misfires

• Poor fuel economy

• Reduced power mode

• Variable valve timing problems

In some cases, the engine may continue to run normally but have a stored P0387 code.

If the crankshaft signal becomes too weak for the ECM to reliably determine crankshaft position, however, the problem can become much more serious.

Can P0387 Cause a No-Start?

Yes.

The crankshaft position signal is often essential for starting the engine.

During cranking, the ECM needs to know when the crankshaft is rotating and where it is positioned. If the signal is too weak or missing, the ECM may be unable to properly synchronize fuel injection and ignition.

The result can be:

• Long cranking

• Intermittent starting

• No-start condition

• Engine stalling immediately after starting

The exact behavior depends on the vehicle's engine management system.

A no-start condition caused by P0387 should therefore not be approached as a simple sensor replacement job. The entire crankshaft position signal circuit needs to be tested.

Common Causes of P0387

Faulty crankshaft position sensor

The sensor itself may have an internal electrical failure.

A defective sensor can produce a signal that is weaker than expected or fail to generate the required waveform.

Heat can also affect some sensors. A sensor may work when cold and produce a weak signal after the engine reaches operating temperature.

Short to ground

One of the most important causes of a low-input code is a signal wire contacting ground.

A damaged harness can allow the signal circuit to touch:

• Engine metal

• Chassis ground

• Another grounded wire

• A damaged connector terminal

This can pull the signal down and cause P0387.

Damaged wiring

The harness can be damaged by:

• Engine vibration

• Excessive heat

• Oil contamination

• Rubbing against metal

• Incorrect routing

• Previous repairs

• Pinching

• Rodent damage

An internally broken conductor can also create a low or unstable signal even when the outside of the insulation looks normal.

Poor connector contact

A loose or corroded connector can increase resistance in the circuit.

Water or oil contamination can also interfere with the electrical connection.

Terminal tension is important. A connector can appear properly plugged in while one terminal has poor contact with the sensor pin.

Sensor ground problem

If the sensor depends on a dedicated ground or ground reference, excessive resistance in that circuit can alter the sensor output.

This is why ground integrity should be tested rather than simply assumed to be good.

Incorrect sensor power supply

On sensor designs that require external power, an incorrect supply voltage can affect the output signal.

The power supply should be measured according to the manufacturer's specifications.

Damaged trigger wheel or reluctor

The crankshaft sensor reads a physical trigger mechanism.

If that mechanism is damaged, loose, bent, contaminated, or incorrectly positioned, the sensor may produce an abnormal signal.

A damaged tooth or abnormal gap can also interfere with crankshaft synchronization.

Incorrect sensor-to-reluctor gap

The distance between the sensor and trigger mechanism can be critical.

If the sensor is incorrectly installed or the trigger wheel has been damaged or displaced, the signal may become weaker than expected.

Metal contamination

Depending on the sensor design, metallic debris near the sensing element can interfere with signal generation.

Oil contamination can also create problems in some applications.

Electrical interference

Improperly routed sensor wiring can be affected by electrical noise from other components.

This is especially worth considering when the fault occurs only at certain engine speeds or under particular electrical conditions.

Mechanical timing problem

A stretched timing chain, damaged timing component, or incorrect mechanical timing can create a mismatch between crankshaft and camshaft signals.

Although this may not directly create a low electrical signal, it can contribute to crankshaft/camshaft synchronization faults and should be considered when sensor and wiring tests are inconclusive.

ECM/PCM fault

An ECM problem is possible but relatively uncommon.

The control module should generally be considered only after the sensor, wiring, power, ground, signal, and trigger mechanism have been properly tested.

How to Diagnose P0387

A systematic diagnosis is much more reliable than immediately replacing the sensor.

1. Scan for all DTCs

Start by checking every stored code.

Pay particular attention to other:

• Crankshaft position codes

• Camshaft position codes

• Synchronization codes

• Misfire codes

• Voltage-related codes

• Communication codes

A combination of codes can reveal whether the problem is isolated to the crankshaft sensor circuit or part of a larger engine-management problem.

2. Check freeze-frame data

Freeze-frame information can show when the ECM detected the low signal.

Look at:

• Engine RPM

• Engine temperature

• Vehicle speed

• Engine load

• Throttle position

• Battery voltage

• Other stored conditions

If P0387 consistently appears during starting, high RPM, or a particular temperature range, this information can help reproduce the fault.

3. Confirm the exact Sensor B circuit

Before testing anything, identify what the manufacturer means by Crankshaft Position Sensor B.

Use the correct wiring diagram and component-location information for that specific vehicle.

4. Inspect the sensor and connector

Check for:

• Corrosion

• Loose terminals

• Bent pins

• Oil contamination

• Water intrusion

• Damaged connector housing

• Poor terminal tension

• Broken locking mechanisms

A connector problem can be enough to create P0387.

5. Inspect the harness

Trace the wiring between the sensor and ECM.

Look for places where the harness could be:

• Rubbing against the engine

• Touching the exhaust

• Pinched

• Melted

• Stretched

• Contaminated

• Incorrectly routed

Pay particular attention to areas exposed to engine movement and vibration.

6. Check the power and ground

For sensors that require external power, verify the supply voltage under the conditions specified by the manufacturer.

Also check the ground circuit.

A voltage-drop test can be more informative than a simple continuity test when a poor connection is suspected.

7. Check for a short to ground

Because P0387 indicates a low signal, the signal wire should be checked for an unintended connection to ground.

The correct procedure depends on the vehicle's circuit design.

Do not apply test voltage to a sensor circuit without following the manufacturer's testing procedure.

8. Monitor the crankshaft signal

A scan tool can provide useful information such as engine RPM during cranking.

If the crankshaft signal disappears or becomes implausible, the RPM reading may drop out.

However, a scan tool may not be fast enough to capture every brief electrical event.

9. Check the waveform

An oscilloscope is extremely useful when the problem cannot be reproduced easily.

A waveform test can reveal:

• Weak signal amplitude

• Missing pulses

• Distorted pulses

• Electrical noise

• Abnormal frequency

• Trigger-wheel irregularities

• Intermittent signal loss

This can help distinguish between a sensor problem and a mechanical trigger problem.

10. Inspect the reluctor

If the electrical circuit is functioning correctly, inspect the crankshaft trigger mechanism.

Look for:

• Damaged teeth

• Missing teeth

• Loose components

• Excessive contamination

• Incorrect positioning

• Physical damage

A sensor cannot produce a correct signal if the mechanism it is supposed to read is damaged.

11. Verify camshaft/crankshaft synchronization

If P0387 appears together with camshaft-related codes or timing symptoms, check synchronization.

A timing-chain or mechanical timing problem can sometimes make a sensor system appear defective when the underlying issue is mechanical.

Why a Sensor Replacement May Not Fix P0387

A crankshaft position sensor is an obvious suspect, but the DTC does not say:

“The crankshaft position sensor is defective.”

It says that the ECM has detected a low input from the Sensor B circuit.

That distinction matters.

For example, suppose the signal wire is partially damaged and intermittently touching the engine block. Replacing the sensor will leave the damaged wire untouched, and P0387 will return.

Likewise, a poor sensor ground, damaged connector, or defective trigger wheel can remain even after installing a brand-new sensor.

This is why the electrical circuit should be tested before replacing expensive components.

P0387 vs. Other Crankshaft Sensor Codes

P0387 belongs to a group of crankshaft position Sensor B codes:

• P0385 – Crankshaft Position Sensor “B” Circuit

• P0386 – Crankshaft Position Sensor “B” Circuit Range/Performance

• P0387 – Crankshaft Position Sensor “B” Circuit Low Input

• P0388 – Crankshaft Position Sensor “B” Circuit High Input

• P0389 – Crankshaft Position Sensor “B” Circuit Intermittent

The distinction is useful during diagnosis.

P0386 indicates that the signal is outside the expected range or performance characteristics.

P0387 indicates a low electrical input.

P0388 indicates a high electrical input.

P0389 indicates an intermittent signal condition.

These codes can have overlapping causes, but their electrical behavior gives the technician a useful direction for testing.

Is P0387 an Electrical or Mechanical Code?

P0387 is primarily an electrical signal code.

That means wiring, connector condition, power, ground, and sensor output should be among the first areas investigated.

However, mechanical problems can still affect the signal indirectly.

For example, a damaged crankshaft reluctor can cause an abnormal sensor output even though the wiring is perfect.

Therefore, the diagnosis should progress from the electrical circuit toward the mechanical trigger system when necessary.

Can You Drive With P0387?

If the engine starts easily, runs normally, and the Check Engine Light is steady, the vehicle may remain driveable for a short period.

However, P0387 should not be ignored.

A crankshaft position signal problem can potentially become a starting or stalling problem without much warning.

Driving is particularly risky if the vehicle has:

• Engine stalling

• Difficult starting

• Sudden power loss

• Severe hesitation

• Repeated no-start conditions

• Major misfires

If the engine stalls in traffic, the loss of propulsion can create a serious safety problem.

A Practical Diagnostic Path

A sensible approach to P0387 is:

P0387 → scan all codes → check freeze-frame data → identify Sensor B → inspect connector and harness → verify power and ground → check for short to ground → monitor crankshaft RPM/signal → test waveform if necessary → inspect trigger mechanism → verify cam/crank synchronization → repair the actual cause → clear the code and retest.

The most important point is that “low input” describes what the ECM is seeing electrically. It does not automatically identify the crankshaft position sensor as the failed component.

A proper diagnosis should determine why the signal is low before parts are replaced.