P0391 — Camshaft Position Sensor B - Range/Performance Problem circuit (bank 2)

Causes

• Faulty camshaft position sensor (Bank 2, sensor B)
• Damaged, corroded or loose wiring/connector to the cam sensor
• Intermittent open, short to voltage or short to ground in sensor circuit
• Failed or sticking variable valve timing (VVT) solenoid or actuator
• Incorrect camshaft/crankshaft timing (worn/jumped timing chain or belt)
• Damaged reluctor/tone wheel or sensor mounting/air gap out of spec

Symptoms

• Check Engine Light (MIL) illuminated, P0391 stored
• Rough idle, misfire or poor running
• Reduced engine power or limp-home mode
• Hard starting or no-start in severe cases
• Poor fuel economy and hesitation on acceleration

What to check

• Read fault code with a scan tool; record freeze frame and pending codes
• Check for related codes (cam/crank correlation, VVT faults, misfire codes)
• Visually inspect sensor connector and wiring for damage, corrosion, pin push-out
• Backprobe sensor connector: verify reference voltage, ground and signal while cranking/at idle
• Use an oscilloscope or graphing scan tool to observe sensor waveform while cranking and at idle/various RPM
• Wiggle test harness to check for intermittent faults while observing live data

Signal parameters

• Hall-effect/three-wire sensor: reference ~5V, signal ~0–5V square wave; frequency increases with RPM
• Two-wire magnetic/VR sensor: AC voltage output; amplitude increases with RPM (millivolts to volts) and is AC waveform
• Expected: clean, consistent pulses with no missing teeth or excessive noise; present during cranking and running
• If available, compare sensor waveform to known-good waveform or to crankshaft position sensor waveform for correlation

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