P0385

Crankshaft Position Sensor B Circuit Malfunction

P0385 is a generic OBD-II powertrain diagnostic trouble code: Crankshaft Position Sensor B Circuit Malfunction. 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.

Code
P0385
Group
Powertrain
System
CKP/CMP
Severity
Warning (MIL on, possible limp mode)
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What P0385 means

P0385 — "Crankshaft Position Sensor \"B\" Circuit Malfunction" — is stored when the PCM/ECM detects that the signal from the secondary (\"B\") crankshaft position sensor circuit is absent or outside of the expected operating range. The crankshaft position sensor (CKP) measures the rotational speed and angular position of the crankshaft by reading a toothed reluctor ring. This information is critical for calculating ignition timing, fuel injection timing, and engine speed — without it, the PCM cannot orchestrate combustion events correctly.

P0385 implies the vehicle is equipped with two crankshaft position sensors, an arrangement used on a number of modern engines for redundancy, high-resolution timing, or torque/knock inference. Engines such as the BMW N57 6-cylinder diesel, Mercedes-Benz OM651, and Ford 6.0L Power Stroke use a dual-CKP strategy where sensor A and sensor B are positioned at different points around the crankshaft or on opposite ends of the crank axis. Sensor A (P0335 family) serves as the primary timing reference; sensor B (P0385 family) provides a cross-check signal or supplies the second sample point needed for full-resolution angular position. When the PCM commands an output on the sensor B circuit and receives no valid pulse train, or detects a signal that is incompatible with the current engine speed from sensor A, P0385 is set. A complete loss of the B signal can cause immediate drivability failure; partial degradation may result in a stored code with no immediate symptom until the condition worsens.

Common causes

Most-frequently reported root causes when P0385 is logged.

  • 1
    Failed crankshaft position sensor \"B\" — internal coil degradation or loss of hall-effect element.
  • 2
    Damaged or missing teeth on the reluctor ring at the sensor \"B\" target zone.
  • 3
    Open circuit, short to ground, or short to battery voltage in the sensor \"B\" signal wiring.
  • 4
    Corroded, damaged, or unseated electrical connector at sensor \"B\" or at the PCM harness pin.
  • 5
    Loss of 5V reference voltage to sensor \"B\" due to a blown fuse or PCM reference circuit fault.
  • 6
    Excessive air gap between sensor \"B\" and the reluctor ring — sensor displaced by impact or incorrect installation.
  • 7
    PCM input circuit fault on the sensor \"B\" channel.
  • 8
    Failed PCM (uncommon — only after all other causes are ruled out).

Symptoms drivers notice

Malfunction Indicator Lamp (MIL / Check Engine Light) illuminates.
Engine may fail to start or crank without firing.
Engine stalling during operation, particularly at idle or low load.
Rough running, misfires, or hesitation as ignition and injection timing becomes unreliable.
Tachometer may display erratic or zero RPM.
Reduced engine power or limp-mode activation.

How to diagnose P0385

A typical diagnostic flow when this code is present.

  1. 1
    Connect an OBD-II scan tool and retrieve all stored and pending codes; note whether sensor A codes (P0335–P0339) or cam sensor codes are also present.
  2. 2
    Inspect the sensor \"B\" connector and wiring harness for corrosion, bent pins, chafing, or moisture intrusion.
  3. 3
    Verify 5V reference voltage and ground integrity at the sensor \"B\" connector with the ignition on.
  4. 4
    Measure sensor \"B\" resistance against manufacturer specifications (typical range 200–1000 ohms for passive sensors).
  5. 5
    Use an oscilloscope to observe the sensor \"B\" waveform during cranking — confirm a consistent square or sine wave with the expected reluctor-gap dropout pattern.
  6. 6
    Inspect the reluctor ring at the sensor \"B\" pickup zone for missing, cracked, or bent teeth.
  7. 7
    Check the sensor mounting position and air gap against manufacturer specifications.
  8. 8
    If wiring, sensor, and reluctor ring are confirmed good, test the PCM input circuit before condemning the module.

Related powertrain codes

Frequently asked questions

Why do some engines have two crankshaft position sensors?

Dual CKP sensors serve several purposes depending on the platform: redundancy (engine keeps running if one sensor fails), higher angular resolution (two sensors at offset positions produce a finer timing grid), or torque inference and knock detection (comparing instantaneous crank velocity between sensors reveals combustion quality per cylinder). BMW N57, Mercedes OM651, and Ford Power Stroke diesels are common examples.

What is the difference between P0335 and P0385?

P0335 covers the primary (\"A\") crankshaft position sensor circuit. P0385 covers the secondary (\"B\") circuit. On vehicles with only one CKP sensor, P0385 will not be present in the calibration. On dual-sensor engines, P0335 typically halts the engine more immediately because sensor A is the primary timing source; P0385 may allow limited operation depending on the calibration.

Can P0385 cause a no-start?

Yes, particularly on platforms where both sensors must provide valid signals before the PCM enables injection. On engines where sensor B is used purely for misfire or knock monitoring, a no-start is less likely but the code is still serious and should be addressed promptly.

Is it safe to drive with P0385?

Driving with this code active is not recommended. The engine may stall without warning, and ignition or injection timing may be compromised. If the engine is currently running and you need to move the vehicle, keep the journey short and drive immediately to a repair facility.

Disabling P0385 in software

RaceTune can permanently disable P0385 — 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.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

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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