What Does P0391 Mean?
The ECM has detected that the camshaft position sensor B signal on bank 2 is outside its expected range or performance parameters. The sensor is producing a signal, but the pattern, amplitude, or timing does not match what the ECM expects based on crankshaft position and engine operating conditions. This may indicate a weak sensor, a slightly jumped timing chain, or a VVT system issue on bank 2. Symptoms include reduced power, slightly rough idle, and potentially decreased fuel economy.
Common Causes
30%
VVT (variable valve timing) actuator or solenoid malfunction on bank 2 camshaft B causing actual cam timing deviation
25%
Timing chain stretch causing cam-to-crank timing to drift beyond the expected window
20%
Weakened CMP sensor B producing inconsistent or low-amplitude signal
15%
Oil viscosity or contamination affecting VVT actuator response and causing timing variation
10%
Worn camshaft reluctor wheel or target pattern affecting signal accuracy
Diagnostic Steps
1
Monitor the CMP sensor B (bank 2) timing advance/retard PID relative to the CKP sensor. Compare the actual camshaft angle to the desired angle. A persistent offset suggests a mechanical timing issue or VVT problem.
2
Check engine oil level and condition. Low oil level or sludged oil can cause VVT actuators to respond slowly or not reach their target position, causing the CMP signal to appear out of range.
3
Capture CMP and CKP waveforms simultaneously on a dual-channel oscilloscope. Measure the timing relationship between the two signals and compare to manufacturer specifications. An offset of more than a few degrees indicates a timing chain or VVT issue.
4
Test the VVT solenoid on bank 2 camshaft B: measure solenoid resistance (typically 7-13 ohms), verify the ECM commands duty cycle changes, and confirm oil flow through the solenoid by actuating it manually with jumper wires.
5
If timing chain stretch is suspected, compare the cam timing offset at idle vs. 3000 RPM. Chain stretch typically causes more offset at idle where chain slack is greatest. Also listen for chain rattle at startup.
Estimated Repair Cost
$100 - $1,500
Parts + labor, varies by vehicle and location
The ECM has detected that the camshaft position sensor B signal on bank 2 is outside its expected range or performance parameters. The sensor is producing a signal, but the pattern, amplitude, or timing does not match what the ECM expects based on crankshaft position and engine operating conditions....
The most common cause of P0391 (Camshaft Position Sensor "B" Circuit Range/Performance (Bank 2)) is: VVT (variable valve timing) actuator or solenoid malfunction on bank 2 camshaft B causing actual cam timing deviation
Typical repair costs for P0391 range from $100 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A range/performance issue means the system is partially functional. The vehicle can typically be driven, but fuel economy and performance are degraded. If timing chain stretch is the cause, continued driving could lead to chain failure and catastrophic engine damage. Diagnose promptly to rule out timing chain issues.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0391 to identify the root cause.
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Quick Info
Category
Powertrain
System
Variable Valve Timing / Engine Timing System
Difficulty
Type
Generic (SAE)
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