Comprehensive Diagnostic Guide for OBD-II Code P0391

Crankshaft Position Sensor Bank B Circuit Range/Performance (CKP Bank B)

Code interpretation and sources context

• Open Source mapping: P0391 is described as a CKP sensor circuit out of range - Bank B sensor. This aligns with the idea that P0391 relates to the crankshaft position sensor circuit and that Bank B is the affected sensor in some vehicle configurations.
  • Source: OBD2 CODE DEFINITIONS .
• General OBD-II context: P0xxx codes are powertrain diagnostic trouble codes; the OBD-II system monitors parameters and reports codes when issues are detected. Wikipedia sections provide broad definitions of DTCs and Powertrain Codes but do not give vehicle-specific definitions for P0391.
  • Source: OBD-II - Diagnostic Trouble Codes
  • Source: OBD-II - Powertrain Codes

Important Notes

• The exact meaning of P0391 can vary by vehicle make/model year. Some OEMs use CKP Bank A vs Bank B designations based on engine architecture (inline vs V-engines, numbering conventions). The Open Source mapping specifically lists Bank B; OEM service information should be consulted for the exact bank designation on a given vehicle.
• If you see P0391, expect a crankshaft position (CKP) signal circuit issue or signal range/performance concern for the designated CKP sensor (Bank B in the mapping you're using). Always cross-check with the vehicle's service information to confirm which CKP sensor is Bank B for that engine.

Typical symptoms reported by drivers (informational summary, not vehicle-specific)

• MIL (Check Engine Light) illuminates with P0391.
• Rough idle or misfire-like symptoms on one side of the engine, particularly when cold or under load.
• Stalling or intermittent no-start conditions.
• Hesitation, reduced power, or poor acceleration until engine speed stabilizes.
• In some cases, multiple CKP-related codes may appear together (e.g., P0391 with other CKP or P0xxx codes).

Probable Causes

• CKP sensor circuit or sensor out of range (Bank B) fault (sensor itself or its signal path): ~40%
• Wiring harness damage, connector corrosion, poor grounds, or damaged shielding in CKP circuit: ~25-30%
• CKP sensor fault vs. reference voltage/ground issues on the PCM side (intermittent or voltage gating problems): ~5-10%
• Engine timing-related mechanical issue (timing belt/chain misalignment, stretched chain, cam/crank timing offset causing CKP signal irregularities): ~10-15%
• Other electrical/ECC or PCM-related faults not directly caused by the CKP sensor (less common): ~5%

Diagnostic Approach

Safety notice: Always follow standard shop safety procedures. Disconnect the battery if you're performing sensor replacement or any wiring repair that requires disconnecting power, and avoid contact with hot surfaces or moving parts. Use proper PPE.

1) Confirm and contextualize the code

• Use a reputable scan tool to confirm P0391 is still present. Note any freeze-frame data and any related codes (e.g., CKP, CAS/CIS, CMP, or P0xxx codes) that appear alongside P0391.
• Check for multiple engine load states in the freeze-frame data (hot vs cold, RPM range, load, ignition timing if available) to understand the operating context when the code was stored.

2) Visual inspection and basic circuit checks

• Inspect the CKP sensor(s) and harness(es) for:
  • Damaged insulation, chafing, or exposed conductors.
  • Loose, corroded, or bent connectors; verify proper locking in the connector.
  • Signs of oil or coolant intrusion, which can contaminate sensor connectors and wiring.
  • Any aftermarket wiring modifications that could alter signal integrity.
• Confirm you are addressing the Bank B CKP circuit as defined for the vehicle (vehicle-specific bank designation varies). If uncertain, consult the OEM service information for which physical sensor is Bank B.

3) Electrical checks on the CKP circuit

• Power/ground validation:
  • With ignition ON (engine off), verify reference voltage to the CKP sensor (often 5 V or 8-12 V depending on design) and check ground continuity.
  • Look for any abnormal voltage drops or noise on the ground that could affect the CKP signal.
• Signal integrity:
  • If accessible, use an oscilloscope to view the CKP waveform from Bank B. Expect a clean, regular waveform corresponding to engine RPM. A distorted, intermittent, or absent waveform indicates wiring issues, a defective sensor, or an ECU/PCM input fault.
  • If you don't have an oscilloscope, a high-quality multimeter with AC coupling or a slow-sampling input may reveal an irregular or intermittent signal; but an oscilloscope is preferred for clear waveform diagnosis.

4) Compare against engine operating state

• Note whether the CKP signal issue occurs at idle, during cranking, or under certain RPM/load conditions. A sensor that only fails intermittently under vibration or at a specific RPM range can indicate a mechanical or connector issue rather than a totally failed sensor.

5) Mechanical and timing considerations

• If electrical tests show a good CKP signal but the engine timing is suspect (e.g., marks out, timing belt/chain slipped, cam timing issue), verify mechanical timing:
  • Inspect timing belt/chain alignment and tension if applicable.
  • Verify that crankshaft and camshaft pulleys and timing marks align per service information.
• Remember that a CKP circuit problem can be caused by a timing issue that disrupts the expected CKP signal pattern, so don't overlook mechanical causes.

6) Cross-check with related systems and codes

• Look for related codes that often accompany CKP faults:
  • CAM or CMP sensor codes (e.g., CMP circuit faults can affect CKP interpretation).
  • Other P0xxx codes related to ignition, misfire, or sensor circuits.
• Review vehicle-specific service information for any known service bulletins (TSBs) related to CKP sensors, wiring harness routing, or ECM/PCM updates that might affect CKP readings.

7) Diagnostic decisions and repair actions

• If the CKP signal is intermittent, distorted, or out of spec and the wiring/connector tests pass:
  • Replace the CKP sensor (Bank B) with a manufacturer-approved part.
  • Re-seat or replace the connector; apply proper dielectric grease if recommended.
  • Use new wiring harness boots if the harness shows signs of damage or if the harness is routed near moving components or hot areas.
• If wiring or connector issues are found:
  • Repair or replace damaged wiring, fix grounds, and ensure secure, clean electrical connections.
• If the CKP signal tests good and timing is correct, but the issue persists:
  • Inspect for mechanical timing problems and correct as needed.
  • Consider PCM/ECU faults if there is evidence of incorrect signal interpretation; reflash or update the ECU if required by OEM procedures.
• After any repair, clear codes and perform a road test or engine run test to verify that P0391 does not reoccur and that no new codes appear.

8) Verification and documentation

• After repairs, re-scan to confirm the code is cleared and monitor live data for the CKP signal (Bank B) during various engine speeds and loads.
• Document the exact sensor part numbers, connector locations, wiring repairs performed, and any timing work completed. Include test results (scope captures or waveform descriptions if available).

When to pursue OEM service information

• If the vehicle shows ambiguity about which CKP sensor is Bank B (or if the engine uses a nonstandard bank designation), consult the OEM service literature for the exact bank designation and signal path. The Open Source P0391 mapping suggests Bank B, but this can vary by manufacturer.

Summary for quick-reference

• Likely cause: CKP sensor B circuit range/performance fault (sensor or wiring).
• Common symptoms: MIL on with rough idle, misfire-like symptoms, stalling, or starting issues.
• Diagnostic flow: confirm code > visual wiring > electrical tests (voltage, ground, signal waveform) > timing/mechanical checks > sensor/connector replacement as needed > verify with road test.
• Key testing tool: oscilloscope for CKP waveform; DVOM for voltage/continuity; battery and ignition-safe test environment.

Cited references

• Wikipedia - OBD-II: Diagnostic Trouble Codes (general context and powertrain scope of DTCs)
  • Notes that DTCs are used by modern automotive systems to monitor parameters and report issues; P0xxx codes are powertrain related (for general code context).
  • Source: OBD-II - Diagnostic Trouble Codes; OBD-II - Powertrain Codes.
• Open Source OBD2 CODE DEFINITIONS: P0391 defined as CKP sensor circuit out of range - Bank B sensor.
  • This provides the practical mapping for Bank B CKP circuit range/performance in the P0391 code.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

---