P0387
Crankshaft Position Sensor B Circuit Low InputP0387 is a generic OBD-II powertrain diagnostic trouble code: Crankshaft Position Sensor B Circuit Low Input. It is logged by the engine control unit when the ckp/cmp monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0387 means
P0387 — "Crankshaft Position Sensor \"B\" Circuit Low Input" — is stored when the PCM detects that the voltage signal from the secondary crankshaft position sensor circuit is continuously at or below the minimum threshold defined in the PCM calibration. Where P0385 indicates a general circuit malfunction, P0387 is directional — the signal is present but stuck low, meaning the PCM is reading a sustained low-voltage condition on the sensor B signal wire rather than the expected alternating pulse train.
A CKP sensor operating normally produces an oscillating signal: a hall-effect type switches between near-0V and the reference voltage (typically 5V or 12V) as each reluctor tooth passes; a passive variable-reluctance type produces a sinusoidal AC voltage. If the signal collapses to near-zero and stays there, it indicates the sensor output transistor is shorted to ground, the signal wire has a fault to ground, or the sensor has failed internally in a way that holds the output low. Because a low-stuck signal looks like an absent or permanent gap to the PCM, the module loses all crankshaft position information and will typically prevent engine starting or cause an immediate stall. On engines where the B sensor is the sole operational CKP (e.g. if sensor A has also failed), a no-start is virtually certain. Vehicles confirmed to use dual CKP sensors include BMW N57/N47, Mercedes OM651, Jeep 3.0L EcoDiesel, Ford 6.0L/6.4L Power Stroke, and various Nissan and GM petrol engines.
Common causes
Most-frequently reported root causes when P0387 is logged.
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1
Short to ground on the sensor \"B\" signal wire — insulation damage, pinched harness, or chafing against a ground point.
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2
Failed crankshaft position sensor \"B\" with an internal short on the output stage holding the signal low.
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3
Corroded or fluid-soaked connector causing the signal pin to be bridged to the ground pin.
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4
Loss of 5V reference supply voltage to the sensor, which causes the output to collapse to ground level.
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5
Broken signal wire that reads as low due to the pull-down characteristics of the PCM input circuit.
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6
Faulty PCM input circuit holding the sensor B channel low internally.
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7
Oil or coolant intrusion into the sensor body or connector causing a conductive short.
Symptoms drivers notice
How to diagnose P0387
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and retrieve all codes; check for companion CKP sensor A codes (P0337 — sensor A low) to determine if the fault is isolated to sensor B or systemic.
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2
Inspect the sensor \"B\" connector, wiring harness, and routing for pinched, chafed, or grounded wiring near frame rails, hot surfaces, or fastener edges.
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3
With the connector unplugged, measure resistance from the signal wire to chassis ground — a reading below 200 ohms confirms a short to ground in the harness.
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4
Verify 5V reference voltage at the sensor connector with the ignition on; an absent or low reference suggests a PCM reference circuit fault or blown fuse.
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5
Measure sensor \"B\" signal voltage with the sensor reconnected and engine cranking — the voltage should oscillate; a flat low voltage confirms the sensor or circuit is stuck low.
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6
Use an oscilloscope on the signal line during cranking to confirm the low-stuck pattern versus a low-amplitude AC signal from a weak variable-reluctance sensor.
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7
If the harness is confirmed intact, replace sensor \"B\" and retest; do not condemn the PCM until sensor and wiring are confirmed serviceable.
Related powertrain codes
- P000A — A Camshaft Position Slow Response Bank 1
- P000B — B Camshaft Position Slow Response Bank 1
- P000C — A Camshaft Position Slow Response Bank 2
- P000D — B Camshaft Position Slow Response Bank 2
- P0010 — A Camshaft Position Actuator Circuit (Bank 1)
- P0011 — A Camshaft Position - Timing Over-Advanced or System Performance (Bank 1)
Frequently asked questions
What is the difference between P0387 and P0388?
P0387 means the crankshaft position sensor B signal is stuck at a low voltage level — typically indicating a short to ground or a failed sensor output pulling low. P0388 is the opposite: the signal is stuck high, suggesting a short to the reference or supply voltage. Both indicate a fixed-voltage signal rather than the expected oscillating waveform.
Can I still drive with P0387?
In most cases no — if the sensor B signal is stuck low, the PCM cannot determine crankshaft position and will withhold injection and/or spark, causing a no-start or immediate stall. If the engine is running at the time of diagnosis, it is likely relying on sensor A alone and the situation is unstable.
How do I distinguish a short-to-ground harness fault from a failed sensor?
Unplug the sensor connector and measure resistance from the signal wire back to chassis ground at the harness side. If the reading is near zero with the sensor disconnected, the fault is in the wiring. If resistance is correct in the harness but the sensor signal is still low when reconnected, the sensor itself has the internal short.
Does P0387 affect sensor A (P0335)?
Not directly — sensor A and sensor B are independent circuits. However, if a wiring fault affects both sensors (e.g. a shorted shared ground or a corroded multi-sensor connector), companion sensor A codes may be present. Always inspect both sensor circuits when P0387 appears alongside sensor A codes.
Disabling P0387 in software
RaceTune can permanently disable P0387 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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