01 What Mechanics Check First When Fuel Usage Suddenly Spikes • 02 Why Does My Check Engine Light Come On After Rain? • 03 Exploring Variable Displacement Engines: Benefits and Drawb… • 04 P0024 B camshaft position - timing over advanced or system … • 05 The Hidden Safety Feature In Modern Cars That Most Owners N… • 06 Why Does My Car Go Faster Than the Speed Limit? • 07 At What Mileage Do Most Cars Start Having Issues? • 08 What Causes the ABS Light to Come On and How to Fix It • 09 Your Beginner’s Guide to Soldering Like a Pro • 10 What Do Pilots Do When One Engine of a Two-Engine Plane Has… •

DTC codes / P03xx · Ignition system and misfire / P0391

P0391Camshaft Position Sensor 'B' Circuit Range/Performance (Bank 2)

P0391 indicates that the powertrain control module (PCM) has detected an out-of-range or erratic signal from the bank 2 camshaft position sensor 'B' circuit, which monitors a second camshaft on that bank. The PCM compares this signal to expected timing from the crankshaft and other cam sensors, and it sets the code when the correlation or signal performance falls outside the programmed limits.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Rough idle or intermittent misfire, especially when the engine is cold
  • Reduced engine power or limp mode under certain driving conditions
  • Poor fuel economy or increased emissions
  • Hard starting or extended cranking before the engine fires
  • Occasional stalling when coming to a stop or during low-speed maneuvers

Common causes, most likely first

  1. Faulty bank 2 camshaft position sensor 'B' (internal failure or contamination)
  2. Damaged or corroded wiring harness or connector at the sensor or PCM
  3. Metal debris or sludge on the sensor tip or reluctor wheel interfering with the signal
  4. Timing chain or belt wear causing the camshaft to deviate from the expected position
  5. PCM software calibration issue or internal PCM fault (less common)
  6. Aftermarket or incorrect sensor installed previously
  7. Oil passage blockage or low oil pressure affecting cam phaser operation (on variable valve timing systems)

How to diagnose and fix it

  1. Connect a scan tool and record freeze-frame data and any stored codes to confirm P0391 is active and not a ghost code
  2. Visually inspect the bank 2 camshaft position sensor 'B' connector and wiring for corrosion, chafing, or loose pins
  3. Measure sensor resistance and check for a clean, consistent signal with an oscilloscope if available
  4. Remove the sensor and inspect the tip and reluctor wheel for metal shavings, sludge, or mechanical damage
  5. Verify base engine timing and cam phaser operation with a scan tool or mechanical timing check
  6. Replace the sensor only after electrical and mechanical checks pass, using an OEM-quality part
  7. Clear codes and perform a road test to confirm the repair, then recheck for pending codes

What the code does not always mean

  • A P0391 is frequently set by a contaminated sensor tip or a failing reluctor wheel rather than a dead sensor, so cleaning and inspection can sometimes resolve it without parts.
  • Wiring harness damage near the exhaust manifold or through a tight routing point is a common hidden cause that a simple sensor swap will not fix.
  • On engines with variable valve timing, a worn timing chain or a sticking cam phaser can produce a P0391 even when the sensor itself tests perfectly.
  • The code can be intermittent and only set during specific conditions such as cold starts or high-load acceleration, so a stored code without a current fault may not indicate a hard failure.
  • A low oil level or degraded oil can affect cam phaser response and trigger this code on some variable valve timing systems, so always check oil condition and level first.

Can I keep driving with P0391?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, towing, or long trips because a failing cam sensor can cause stalling or limp mode without warning. If the engine runs rough, misfires, or stalls, stop driving and arrange a tow to prevent catalytic converter damage or unsafe driving conditions.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage