P0391 — Camshaft Position Sensor "B" Circuit Range/Performance (Bank 2)

What does P0391 mean on this engine?

P0391 means the control module found the Bank 2 camshaft position sensor B signal outside its expected range or inconsistent with the crank reference. The fault may involve the sensor, wiring, reluctor, oil delivery, or cam timing.

Can I continue driving with P0391 stored?

Only briefly if the engine runs smoothly and without stalling. P0391 can cause a loss of power or failure to start, so diagnosis is recommended before normal driving, especially if the code is pending or accompanied by an oil-pressure warning.

Does P0391 always require a camshaft sensor replacement?

No. Before replacing the sensor, verify connector condition, reference voltage, ground integrity, signal stability, oil pressure, and cam timing. A wiring defect, damaged reluctor, or low-pressure timing fault can produce the same P0391 code.

Why does P0391 return after the sensor is replaced?

A replacement sensor will not correct P0391 if the underlying issue is corroded wiring, a poor ground, a damaged reluctor, restricted oil flow, or incorrect cam timing. Confirm the signal waveform and mechanical timing before replacing another component.

When your engine control unit stores trouble code P0391, it's flagging a problem with the camshaft position sensor circuit on the exhaust side of the engine (Bank 2). The PCM constantly monitors signals from this sensor to determine where each camshaft sits in its rotation cycle, information it needs to synchronize fuel injection and ignition timing with valve events. If the signal falls outside expected parameters or behaves inconsistently, the module detects the anomaly and logs P0391 alongside a freeze frame snapshot capturing engine speed, load, temperature, and vehicle speed at the moment of failure.

Modern engines use variable valve timing (VVT) to optimize performance across different operating conditions. The camshaft position sensor provides the critical feedback that allows the PCM to command the VVT system—whether through hydraulic solenoid control or electric actuators—to advance or retard cam timing. When this feedback loop breaks down, either through sensor failure, mechanical wear in the timing system, or restricted oil flow to the VVT actuator, the engine may run poorly, hesitate, or fail to start. The ECU sets P0391 after confirming the abnormality across multiple drive cycles, not just from a single glitch, which is why the code often persists after clearing and requires proper diagnosis rather than just replacement.

Common Symptoms of P0391

The engine may idle roughly or feel sluggish during low-speed acceleration. In more serious cases, especially if the VVT system has failed entirely or timing has jumped, the engine may not start at all or require extended cranking. The Check Engine light will illuminate, and drivers typically notice reduced power under load. Fuel economy often worsens because the engine runs in an inefficient operating mode. Symptoms vary depending on whether the fault is electrical (sensor or wiring) or mechanical (timing or VVT actuator).

Common Causes

Low or contaminated engine oil is one of the most frequent culprits because VVT systems depend on clean, properly pressurized oil to actuate phasers and solenoid valves. A stuck or clogged VVT oil control valve (OCV) can prevent proper cam phase adjustment. Worn timing chain components or a jumped timing belt will cause the cam position signal to disagree with what the PCM expects. Physical damage to the sensor wiring, connectors, or the reluctor ring (tone wheel) on the cam shaft can corrupt the signal. A failing camshaft position sensor itself may produce intermittent or out-of-spec readings. Finally, a worn cam phaser with excessive internal leakage can prevent the VVT system from achieving commanded positions.

How to Diagnose P0391

Begin by connecting an OBD2 scanner to confirm P0391 is stored and record the freeze frame data—this tells you engine rpm, load, coolant temperature, and vehicle speed when the fault occurred, guiding your inspection. Check for related trouble codes that often point to the root cause; for example, multiple misfire codes alongside P0391 might suggest a timing problem rather than a simple sensor failure. Review any applicable technical service bulletins for your make and model, as manufacturers may have published fixes, software updates, or known failures for this specific code. Visually inspect the wiring harness and connectors at the camshaft position sensor and VVT solenoid for chafing, corrosion, or loose pins. Check engine oil level and condition—dark, sludgy oil or low levels directly affect VVT operation. Compare live cam position data (commanded versus actual) using your scanner while gently revving the engine, watching for delayed or insufficient response. After repairs or inspections, clear the code and complete a drive cycle matching the freeze frame conditions to verify the fault does not return.

Repair Options and Costs

If the VVT oil control valve is stuck or clogged, replacing it typically costs between $200 and $500 for parts and labor. When the timing chain or belt has jumped or worn excessively, repair generally runs $800 to $2,500 depending on engine design and labor rates. A faulty camshaft position sensor replacement usually falls between $120 and $350 installed. In some cases, wiring repair or connector replacement resolves the issue at minimal cost.

Total Repair Cost for P0391

The wide range reflects how many different components can trigger this code. Expect to pay anywhere from $120 to $2,500 depending on which part actually failed. Professional diagnosis before purchasing parts almost always saves money, since the most expensive repair scenario (timing chain work) should not be the first assumption when the sensor itself may be the problem.

Can You Drive with P0391?

Drive with caution. The engine may run in a reduced-power failsafe mode, and if timing has jumped on an interference engine, continued operation risks contact between valves and pistons—potentially catastrophic damage. Diagnose the underlying cause promptly and avoid long trips until the issue is resolved.

Related OBD2 Codes

• P0300: Random/Multiple Cylinder Misfire Detected
• P0301: Cylinder 1 Misfire Detected
• P0302: Cylinder 2 Misfire Detected
• P0303: Cylinder 3 Misfire Detected
• P0304: Cylinder 4 Misfire Detected
• P0305: Cylinder 5 Misfire Detected