Camshaft Position Sensor B Circuit Range/Performance (Bank 2)
The P0391 code means the vehicle's onboard diagnostic system has detected: Camshaft Position Sensor B Circuit Range/Performance (Bank 2). This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- moderate
- Estimated cost
- $60 - $600 (parts and labor)
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Common Symptoms
- Check Engine Light on
- Rough idle or hesitation
- Reduced performance
- Possible timing chain noise
Probable Causes (Ranked by Likelihood)
- Degraded cam sensor B signal quality Very High
- Timing chain stretch on Bank 2 High
- Incorrect sensor air gap Moderate
- PCM calibration issue Low
Step-by-Step Diagnostic Procedure
-
Compare cam sensor B signal to spec on oscilloscope.
-
Check timing chain slack on Bank 2.
-
Verify sensor air gap.
-
Compare Bank 1 and Bank 2 sensor signals.
Common Fixes by Vehicle
What techs usually find when diagnosing P0391 on specific vehicles — tap your vehicle for the fix and the exact part:
2005-2015 Nissan Altima/Maxima/Pathfinder VQ35/VQ40 Moderate DIY
Bank 2 (passenger side) camshaft position sensor B circuit fault. Replace the CMP sensor on the rear of the passenger-side cylinder head. Use Nissan OEM or Hitachi unit; generic sensors commonly re-trigger within weeks.
Labor: 45-60 min2004-2014 Ford F-150/Explorer/Mustang 4.0L/3.7L V6 Moderate DIY
Bank 2 cam sensor B circuit malfunction — replace the cam position sensor on the passenger side head. On 4.0L SOHC, also inspect for timing chain stretch (a well-known 4.0L SOHC failure).
Labor: 45 min-1 hr2007-2013 Infiniti G35/G37/FX35 VQ35/VQ37 Moderate DIY
Bank 2 CMP-B fault — replace cam sensor with OEM Nissan/Hitachi. Inspect wiring harness under the intake for damage from rodent chewing (common VQ35 complaint). If chain stretch is suspected via long crank times, timing chain service is due.
Labor: 1-1.5 hrFrequently Asked Questions
Can timing chain stretch cause P0391?
Yes, a stretched timing chain alters the cam-to-crank relationship, causing the sensor signal to fall outside the expected range.
Is P0391 a sensor or mechanical problem?
It can be either. Start by checking the sensor, but if the sensor is good, investigate the timing chain.
How do I check for timing chain stretch?
Use an oscilloscope to compare crank and cam sensor waveform timing, or check for chain slack with a tensioner gauge.
Sources
This page is built from documented references. Verify against your own service info before repair work.
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