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

What it means

Cam sensor B on bank 2 reads in range but plausibility check fails — slow response, drift, or signal does not match expected cam profile.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

Camshaft position sensor B reports the position of the B camshaft, commonly the exhaust cam, on bank 2. The engine control module checks whether the signal changes at the expected rate and agrees with engine timing. This code means the signal is present but its response or relationship to expected cam position is implausible.

Common causes

Most likely first.

1. Sensor drift
2. Sensor element aged beyond calibration window
3. Sensor mounting loose - signal damping

Symptoms

• Check engine light on
• Power loss especially in mid-rpm range
• Possible rough idle or stalling
• Increased fuel consumption

How to diagnose it

1. Record freeze-frame data and review live bank 2 sensor B cam position, engine speed, and synchronization readings for drift or delayed changes.
2. Inspect sensor B on bank 2 and its connector for a loose mounting, poor pin contact, contamination, or damaged wiring.
3. Check the sensor’s power, ground, and signal circuit using the wiring diagram and manufacturer’s procedure.
4. Compare the bank 2 B camshaft signal with crankshaft position and the other camshaft readings to identify a timing mismatch.
5. If the circuit is sound, test sensor response and inspect the camshaft timing system for wear or incorrect timing.

Common fixes

• Secure a loose bank 2 sensor B mounting.
• Repair damaged wiring or poor connector contact in the sensor circuit.
• Replace sensor B on bank 2 if its response is outside the manufacturer’s specification.
• Correct a confirmed camshaft timing problem.

Can I keep driving?

Short trips may be possible if the engine runs smoothly, but avoid towing or hard acceleration because power may be reduced. Stop driving if it stalls or runs severely rough, since incorrect cam timing can affect engine operation.