P0391 indicates the powertrain control module (PCM) has detected an abnormal signal from the camshaft position sensor 'B' on bank 2. This sensor monitors the camshaft's position and speed to help synchronize fuel injection and ignition timing. A performance issue suggests the signal is outside expected parameters but not completely missing.
The sensor itself may have failed due to internal wear, damage, or contamination from oil or debris. This is the most common cause of P0391.
Damaged, corroded, or loose wiring or connectors in the sensor circuit can cause intermittent or out-of-range signals. Common areas include near the sensor or along the harness.
If the sensor is not properly aligned with the reluctor wheel or the gap is incorrect, the signal may be weak or erratic, triggering the code.
A stretched timing chain, jumped timing, or damaged reluctor wheel can cause the camshaft to be out of sync with the crankshaft, leading to an out-of-range signal.
Oil, dirt, or metal debris on the sensor or reluctor wheel can interfere with the magnetic field, causing a weak or inconsistent signal.
While uncommon, a PCM software glitch or internal failure could incorrectly interpret the sensor signal. This is a last-resort diagnosis after all other causes are ruled out.
No, the PCM is rarely the cause of P0391. The issue is almost always related to the camshaft position sensor itself, its wiring, or the timing system. PCM replacement should only be considered after all other potential causes have been thoroughly ruled out.
While the vehicle may still run, driving with P0391 can lead to poor engine performance, reduced fuel economy, or potential engine damage if the issue is timing-related. It’s best to diagnose and repair the problem as soon as possible.
Bank 2 refers to the side of the engine that does not contain cylinder #1. The code indicates the issue is isolated to the camshaft position sensor on that bank. If the sensor or its circuit fails, the PCM may not properly control fuel injection or ignition timing for that bank.
No, P0391 is specific to the camshaft position sensor 'B' on bank 2. However, a failing crankshaft position sensor can cause similar drivability issues and may set its own trouble codes, so it should be checked if other codes are present.