Crankshaft Position Sensor B Circuit Low
The P0387 code means the vehicle's onboard diagnostic system has detected: Crankshaft Position Sensor B Circuit Low. This is a critical severity code.
- Keep driving?
- No — stop driving
- DIY difficulty
- moderate
- Estimated cost
- $80 - $400 (parts and labor)
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Common Symptoms
- Check Engine Light on
- No-start or stalling
- Erratic RPM
- Loss of power
Probable Causes (Ranked by Likelihood)
- Short to ground in crank sensor B wiring Very High
- Failed sensor (shorted) High
- Corroded connector Moderate
- PCM input shorted Low
Frequently Asked Questions
What causes circuit low on crank sensor B?
A short to ground in the wiring or a failed sensor with low output voltage.
Can I diagnose this without an oscilloscope?
Basic diagnosis can be done with a multimeter to check resistance and shorts, but an oscilloscope is needed to verify signal quality.
Is this code common on diesel trucks?
Yes, diesel engines with dual crank sensors are more likely to set sensor B codes.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
-
Measure crank sensor B signal voltage. Below spec at running RPM indicates low circuit.
-
Disconnect sensor and check for short to ground.
-
Inspect connector for moisture or corrosion.
-
Compare to sensor A readings.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
- Cross-checked against the SAE J2012:2002 DTC table (2026-08-27)archived scan of the SAE standard — readable copy, not SAE’s current edition
Commonly Affected Vehicles
- 2010-2020 Ford F-250 (6.7L Diesel)
- 2012-2019 Ram 2500 (6.7L Cummins)
- 2008-2018 Chevrolet Silverado 2500 (6.6L Duramax)
- 2011-2020 GMC Sierra 2500 (6.6L Duramax)
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