The P0391 OBD-II trouble code means the powertrain control module is receiving a signal from Camshaft Position Sensor “B” on Bank 2, but that signal falls outside the expected operating range or does not perform as expected.
Unlike P0390, which identifies a general circuit malfunction or unusable signal, P0391 usually means the sensor is producing a waveform. The problem is that its amplitude, pulse spacing, timing or relationship to other engine-position signals is implausible.
A weak camshaft-position sensor can cause P0391, but so can excessive sensor clearance, a damaged reluctor, electrical interference, dirty engine oil, a sticking camshaft phaser or incorrect mechanical timing. Replacing the sensor without inspecting its waveform may solve the problem. It may also produce the familiar combination of a new sensor and an old code.
P0391 Quick Reference
| Item | Details |
|---|---|
| Trouble code | P0391 |
| Code definition | Camshaft Position Sensor “B” Circuit Range/Performance Bank 2 |
| System affected | Bank 2 camshaft-position monitoring |
| Likely sensor | Usually the Bank 2 exhaust camshaft sensor |
| Fault type | Signal exists but is implausible or outside the expected range |
| Common causes | Weak sensor, excessive air gap, reluctor damage, wiring noise, VVT problems or incorrect timing |
| Common symptoms | Check Engine Light, rough idle, hesitation, hard starting or reduced power |
| Typical severity | Moderate |
| Safe to drive? | Possibly for a short distance if the engine runs normally |
| Repair difficulty | Intermediate to advanced |
What Does the P0391 Code Mean?
P0391 indicates that the PCM can detect electrical activity from Camshaft Position Sensor B on Bank 2, but the signal does not match the engine’s expected operating conditions.
The PCM may store P0391 when it detects:
- Incorrect waveform amplitude
- Uneven pulse spacing
- Missing or additional pulses
- A distorted reference pulse
- Signal frequency that does not match engine speed
- Incorrect Sensor B timing
- Poor waveform quality
- Electrical noise
- An implausible relationship with another camshaft sensor
- Sensor output that becomes unreliable at certain RPM
- A signal that does not follow commanded camshaft movement
The exact code-setting criteria vary by manufacturer. Some PCMs focus primarily on electrical waveform quality, while others also evaluate Sensor B operation relative to variable-valve-timing commands.
P0391 should therefore be diagnosed with service information, scan data and waveform analysis rather than a generic sensor resistance specification downloaded from the internet’s most confident stranger.
What Does Sensor B Mean?
On many dual-overhead-cam engines:
- Sensor A monitors the intake camshaft
- Sensor B monitors the exhaust camshaft
This convention is common but not universal. Manufacturers may identify Sensor B according to circuit function or physical location.
Before diagnosis, confirm:
- Whether Sensor B monitors the intake or exhaust camshaft
- Its physical location
- Sensor type
- Connector pinout
- Power and ground requirements
- Expected waveform
- Specified sensor air gap
The article uses “exhaust camshaft sensor” as the common application, but the vehicle’s service information remains authoritative.
What Does Bank 2 Mean?
Bank 2 is the cylinder bank that does not contain cylinder number one.
It is not always the driver side or passenger side. Bank location depends on engine design and vehicle installation.
P0391 normally applies to V-type or opposed engines with two cylinder banks. An inline engine generally has only Bank 1 and will not use a Bank 2 code.
Confirm Bank 2 through:
- Cylinder numbering
- Firing-order information
- Manufacturer component diagrams
- Factory service procedures
How the Camshaft-Position Signal Works
The camshaft-position sensor monitors a toothed, slotted or magnetized trigger wheel attached to the camshaft, camshaft actuator or phaser.
As the camshaft rotates, the sensor generates a waveform. The PCM uses this information to determine:
- Cylinder position
- Sequential fuel-injection timing
- Ignition timing
- Camshaft position
- Variable-valve-timing operation
- Startup synchronization
- Misfire-monitoring accuracy
The PCM compares Sensor B with the crankshaft-position signal and, on many engines, the other camshaft sensors.
When the signal remains electrically present but no longer looks believable, P0391 may be stored.
Hall-Effect and Magnetic Camshaft Sensors
Two common sensor designs are used.
Hall-Effect Sensor
A Hall-effect sensor generally has three circuits:
- Power or reference voltage
- Sensor ground
- Digital signal output
The signal switches between defined high and low voltage states as the camshaft trigger passes the sensor.
Performance may be affected by:
- Unstable reference voltage
- Poor ground
- Excessive air gap
- Damaged trigger wheel
- Weak internal electronics
- Wiring resistance
- Electrical interference
- Incorrect sensor installation
Magnetic Sensor
A magnetic or variable-reluctance sensor usually has two wires and generates an alternating-current waveform.
Its output depends on:
- Camshaft speed
- Sensor air gap
- Sensor magnet strength
- Reluctor condition
- Circuit resistance
- Temperature
A magnetic sensor may produce a waveform that is technically present but too weak or distorted for the PCM to interpret reliably.
Symptoms of a P0391 Code
Possible symptoms include:
- Illuminated Check Engine Light
- Extended cranking
- Hard starting
- Rough idle
- Engine hesitation
- Reduced power
- Poor throttle response
- Misfire-like operation
- Engine stalling
- Increased fuel consumption
- Variable valve timing disabled
- Limp-home mode
- Unstable camshaft-position data
- Difficult hot or cold starting
- No noticeable driveability symptoms
Some vehicles continue operating using a default camshaft position or information from other engine-position sensors.
If the engine turns over but will not start, use the engine cranks but won’t start diagnostic guide to check spark, injector command, fuel pressure and synchronization data.
Common Causes of P0391
Potential causes include:
- Weak Camshaft Position Sensor B on Bank 2Internal sensor deterioration may produce a waveform with incorrect amplitude or switching quality.
- Excessive Sensor Air GapToo much clearance between the sensor and trigger wheel can weaken the signal.
- Incorrect Sensor InstallationDebris beneath the mounting flange, an incorrect spacer or improper seating may alter the air gap.
- Damaged Camshaft ReluctorBent, missing, cracked or contaminated trigger features can distort the waveform.
- Loose or Shifted ReluctorA trigger wheel that moves relative to the camshaft can change the signal timing.
- High Resistance in the Signal CircuitCorrosion, loose terminals or damaged wiring can weaken the waveform before it reaches the PCM.
- Unstable Sensor Power or GroundVoltage fluctuations may distort an active sensor’s output.
- Electrical InterferenceDamaged shielding or improper harness routing can introduce false pulses and noise.
- Oil-Contaminated ConnectorEngine oil inside the connector can alter circuit resistance or bridge terminals.
- Incorrect Replacement SensorA sensor with the wrong electrical characteristics or mounting depth may produce an unacceptable waveform.
- Dirty, Low or Incorrect Engine OilOil condition can affect the Bank 2 variable-valve-timing actuator and camshaft response.
- Sticking Bank 2 Camshaft PhaserA phaser that does not move consistently may cause actual camshaft position to disagree with the expected response.
- Restricted VVT Oil PassageSludge or debris may prevent proper actuator movement.
- Faulty VVT Control SolenoidAn exhaust-camshaft control solenoid may command the phaser incorrectly or respond too slowly.
- Incorrect Mechanical TimingA stretched timing chain, jumped timing belt or incorrect installation can shift Sensor B timing.
- Excessive Camshaft End PlayCamshaft movement can change the sensor-to-reluctor relationship.
- PCM Software or Input ProblemA software calibration or internal input fault may cause the PCM to misinterpret an otherwise usable signal.
How Serious Is P0391?
P0391 is usually a moderate-severity fault.
The vehicle may continue running, but inaccurate Bank 2 camshaft-position data can affect:
- Starting
- Fuel-injection timing
- Ignition timing
- Variable-valve-timing operation
- Fuel economy
- Engine power
- Emissions
- Misfire monitoring
Diagnosis should be performed promptly if the engine runs poorly, stalls or produces timing-chain noise.
A vehicle with severe mechanical noise, heavy misfiring or a flashing Check Engine Light should not be driven.
P0390 vs. P0391
P0390 and P0391 affect the same sensor but describe different fault conditions.
| Code | Definition | Primary diagnostic focus |
| P0390 | Sensor B Circuit Malfunction Bank 2 | General electrical failure or unusable signal |
| P0391 | Sensor B Circuit Range/Performance Bank 2 | Signal exists but its waveform or response is implausible |
P0390 points more strongly toward a dead circuit, open wire, short or missing sensor supply.
P0391 requires closer inspection of waveform shape, sensor clearance, trigger-wheel condition and camshaft response.
P0391 vs. P0366
P0366 is the corresponding Sensor B range/performance code for Bank 1.
| Code | Bank | Sensor | Fault |
| P0366 | 1 | Sensor B | Range/performance |
| P0391 | 2 | Sensor B | Range/performance |
If both codes are stored, inspect shared power, ground, oil condition, mechanical timing and PCM circuits before assuming that both sensors failed simultaneously.
P0391 vs. P0346
P0346 applies to Camshaft Position Sensor A on Bank 2. P0391 applies to Sensor B on Bank 2.
On a typical DOHC engine:
- P0346 concerns the Bank 2 intake camshaft sensor
- P0391 concerns the Bank 2 exhaust camshaft sensor
Confirm the design through service information.
P0391 vs. P0019
P0019 specifically identifies incorrect correlation between the crankshaft and Bank 2 Sensor B.
P0391 is broader. It means the Sensor B signal or response falls outside its expected performance range.
The two codes may appear together, but they retain different search and diagnostic intent:
- P0391 owns Sensor B waveform and response quality
- P0019 owns measured crankshaft-to-camshaft timing correlation
P0391 vs. Bank 2 VVT Timing Codes
Bank 2 Sensor B timing codes include:
P0391 should not be optimized as an over-advanced or over-retarded timing article. Those codes own those specific timing conditions.
How to Diagnose the P0391 Code
P0391 diagnosis should focus on waveform quality and actual camshaft response. A static resistance test cannot reveal incorrect pulse timing, excessive air gap or a phaser that reacts too slowly.
1. Confirm P0391
Connect a capable OBD-II scan tool and record:
- P0391
- Current, pending and history codes
- Freeze-frame information
- Engine RPM
- Battery voltage
- Engine coolant temperature
- Vehicle speed
- Engine load
- Desired Bank 2 Sensor B cam angle
- Actual Bank 2 Sensor B cam angle
- Camshaft synchronization
- Crankshaft synchronization
- VVT command or duty cycle
Do not clear the code before recording the data.
2. Check for Related Codes
Look for codes involving:
- P0390 through P0394
- Bank 1 Sensor B
- Bank 2 Sensor A
- Crankshaft-position sensors
- Crank-to-cam correlation
- VVT control solenoids
- Camshaft timing
- Oil-pressure control
- Sensor reference voltage
- Engine misfires
Related codes can reveal whether the problem is electrical, mechanical or oil-control related.
3. Identify Bank 2 and Sensor B
Use factory service information to confirm:
- Cylinder number-one location
- Bank 2 location
- Sensor B location
- Intake or exhaust designation
- Sensor type
- Connector pinout
- Expected signal voltage
- Sensor air-gap specification
- Expected waveform
Do not begin diagnosis until the correct sensor is identified.
4. Check Engine Oil
Verify:
- Oil level
- Oil condition
- Correct viscosity
- Correct oil specification
- Service interval
- Signs of sludge
- Signs of fuel or coolant contamination
Low, dirty or incorrect oil can interfere with VVT actuator response.
Correct an obvious oil problem before condemning the camshaft sensor or phaser.
5. Inspect the Sensor and Connector
Check for:
- Oil inside the connector
- Corroded terminals
- Loose terminal tension
- Bent or pushed-back pins
- Broken connector locks
- Damaged sensor housing
- Incorrect mounting
- Debris beneath the mounting flange
Oil contamination should be traced to its source. Installing a new sensor while leaving its connector full of oil is not a repair plan so much as a recurring subscription.
6. Inspect the Wiring Harness
Follow the circuit toward the PCM.
Look for:
- Melted insulation
- Harness contact with exhaust components
- Wiring pinched beneath valve covers
- Stretched conductors
- Damaged shielding
- Improper splices
- Wiring damage from prior timing repairs
- Harness movement near engine mounts
- Oil-soaked insulation
Move the harness carefully while monitoring scan data or waveform stability.
7. Check Battery and Charging Voltage
Test:
- Battery state of charge
- Minimum cranking voltage
- Engine-ground voltage drop
- PCM-ground voltage drop
- Charging voltage
- Alternator ripple
Unstable system voltage can distort active-sensor output and PCM interpretation.
8. Verify Sensor Power and Ground
For a Hall-effect or other active sensor, verify:
- Specified supply voltage
- Ground voltage drop
- Signal bias voltage
Test the circuits with the sensor connected whenever the approved procedure allows it. A supply may measure correctly unloaded and collapse when the sensor is connected.
Repair unstable power or ground before interpreting the signal waveform.
9. Capture the Sensor B Waveform
Use an oscilloscope to inspect Bank 2 Sensor B during:
- Cranking
- Idle
- Acceleration
- Deceleration
- Warm operation
- The RPM recorded in freeze-frame data
Inspect:
- High-state voltage
- Low-state voltage
- Waveform amplitude
- Pulse width
- Pulse spacing
- Reference features
- Electrical noise
- Missing or additional pulses
A P0391 waveform may remain present but show weak amplitude, distorted edges or inconsistent spacing.
10. Compare Bank 1 and Bank 2 Sensor B
Display both Sensor B signals simultaneously.
Compare:
- Voltage levels
- Pulse pattern
- Signal stability
- Timing relative to the crankshaft
- Response to VVT commands
If Bank 1 is clean and Bank 2 is distorted, concentrate on the Bank 2 sensor, wiring, reluctor and phaser.
If both banks behave incorrectly, inspect shared power, ground, oil supply, PCM and base timing.
11. Compare the Camshaft and Crankshaft Signals
Display Bank 2 Sensor B alongside the crankshaft-position signal.
Check whether:
- The pulse pattern is stable
- The reference relationship changes unexpectedly
- Bank 2 timing shifts during operation
- The waveform contains repeatable defects
- The camshaft moves in response to PCM commands
A waveform with correct electrical levels but incorrect timing points away from a simple sensor circuit and toward a reluctor, phaser or mechanical-timing problem.
12. Compare the Signal at the Sensor and PCM
Capture Sensor B at its connector and then at the PCM.
If the waveform is correct at the sensor but distorted at the PCM, inspect for:
- High circuit resistance
- Loose connector terminals
- Damaged shielding
- Electrical interference
- Shorting between circuits
- Corroded splices
If the waveform reaches the PCM correctly but the scan data remains implausible, investigate PCM power, ground, software and input operation.
13. Check Sensor Air Gap
Measure the sensor-to-reluctor clearance when the design permits it.
Excessive or inconsistent clearance may result from:
- Debris beneath the sensor
- Incorrect sensor length
- Improper installation
- Damaged mounting surface
- Camshaft end play
- Reluctor runout
- Loose trigger components
Do not move the sensor closer than specified. Contact between the sensor and rotating trigger wheel converts an air-gap issue into a parts-replacement issue surprisingly quickly.
14. Inspect the Reluctor
Check for:
- Bent trigger tabs
- Missing trigger features
- Cracks
- Loose mounting
- Oil sludge or metal debris
- A shifted trigger wheel
- Physical damage from previous repairs
Internal reluctors may require a borescope or waveform diagnosis before disassembly.
A repeatable defect at the same point in every camshaft revolution strongly suggests trigger-wheel damage.
15. Command the VVT System
If the scan tool supports bidirectional controls, command the Bank 2 Sensor B camshaft actuator while monitoring:
- Desired camshaft angle
- Actual camshaft angle
- VVT solenoid duty cycle
- Oil pressure data, when available
- Sensor B waveform
The actual camshaft position should follow the commanded value smoothly and within manufacturer limits.
Possible results include:
- Actual angle does not move: oil supply, solenoid or phaser problem
- Actual angle moves slowly: restricted oil passage or sticking phaser
- Actual angle overshoots: actuator-control or phaser problem
- Scan angle jumps while the physical waveform remains stable: signal interpretation or wiring problem
- Waveform becomes distorted only during movement: reluctor, air-gap or sensor problem
16. Inspect the VVT Solenoid
If commanded and actual positions disagree, inspect the Bank 2 Sensor B camshaft-control solenoid.
Check for:
- Electrical circuit codes
- Correct resistance when specified
- Mechanical sticking
- Sludge
- Blocked screens
- Restricted oil passages
- Incorrect installation
Do not replace the VVT solenoid solely because P0391 is present. The waveform and command data should support that direction.
17. Verify Mechanical Timing
Investigate timing-chain or timing-belt alignment when:
- Crank-to-cam comparison is incorrect
- Correlation codes accompany P0391
- Both cam sensors show shifted timing
- The engine produces mechanical noise
- Recent timing repairs were performed
- The phaser reaches a mechanical limit
- Desired and actual positions remain consistently offset
Check:
- Timing marks
- Chain stretch
- Belt indexing
- Tensioner operation
- Guide condition
- Phaser position
- Reluctor alignment
Mechanical timing should be verified through evidence, not used as the opening move for every camshaft-related code.
18. Check for Heat-Related Failure
Monitor the waveform as the engine reaches operating temperature.
Look for:
- Reduced signal amplitude
- Slower switching
- Increased electrical noise
- Camshaft response changing when hot
- Sensor operation recovering after cooling
A sensor, solenoid or phaser may behave normally cold and fail after the oil and engine reach operating temperature.
19. Consider PCM Software or Failure Last
Before replacing the PCM, confirm:
- Correct Sensor B identification
- Correct Bank 2 identification
- Stable sensor power and ground
- Good waveform at the PCM
- Correct oil level and viscosity
- Good reluctor condition
- Proper VVT response
- Correct mechanical timing
- No applicable calibration update or technical service bulletin
PCM replacement may require programming, security initialization and camshaft or crankshaft relearn procedures.
Common P0391 Diagnostic Mistakes
Avoid these common mistakes:
- Confusing Sensor A with Sensor B
- Testing Bank 1 instead of Bank 2
- Assuming Sensor B is always the exhaust sensor
- Replacing the sensor without viewing the waveform
- Ignoring oil level and viscosity
- Treating P0391 like the dead-circuit P0390 code
- Replacing a timing chain before comparing cam and crank signals
- Performing only a resistance test
- Ignoring excessive sensor air gap
- Overlooking reluctor damage
- Replacing the VVT solenoid without commanding the system
- Failing to compare desired and actual camshaft angles
- Replacing the PCM before checking the signal at its connector
Repairs That May Fix P0391
Possible repairs include:
- Replacing a weak Bank 2 Camshaft Position Sensor B
- Cleaning or replacing the sensor connector
- Repairing high resistance in the signal circuit
- Repairing sensor power or ground
- Restoring damaged waveform shielding
- Correcting harness routing
- Installing the correct sensor
- Correcting sensor air gap
- Repairing or replacing a damaged reluctor
- Changing contaminated or incorrect engine oil
- Cleaning restricted VVT oil passages
- Replacing a faulty Bank 2 Sensor B VVT solenoid
- Replacing a sticking camshaft phaser
- Correcting mechanical timing
- Replacing a stretched timing chain or damaged guides
- Updating PCM software
- Replacing and programming the PCM after confirmed failure
Estimated P0391 Repair Costs
Actual costs depend on the vehicle and root cause.
| Repair | Estimated cost |
| Diagnostic inspection | $120–$300 |
| Oil and filter service | $80–$200 |
| Connector or minor wiring repair | $100–$400 |
| Camshaft Position Sensor B replacement | $180–$550 |
| VVT control-solenoid replacement | $200–$700 |
| Major harness repair | $350–$1,200 |
| Camshaft-phaser replacement | $800–$2,500 |
| Timing-chain repair | $1,000–$4,000 |
| PCM software update | $100–$300 |
| PCM replacement and programming | $800–$2,500 |
The sensor may be inexpensive. Discovering that its waveform is accurately reporting a worn phaser or stretched timing chain is where the repair estimate begins developing ambition.
Can You Drive With P0391?
A short drive may be possible if the engine starts easily, runs smoothly and produces no mechanical noise.
Do not continue driving if:
- The engine stalls
- Starting becomes unreliable
- The engine misfires heavily
- The Check Engine Light flashes
- Power drops significantly
- Timing-chain noise develops
- Oil pressure is low
- Multiple correlation or timing codes appear
If the Check Engine Light behavior is unclear, review what the Check Engine Light means.
How to Verify the Repair
After completing the repair:
- Reconnect all sensors and modules.
- Confirm proper connector engagement.
- Verify the engine-oil level.
- Clear current and pending trouble codes.
- Monitor Bank 2 Sensor B during cranking.
- Compare Bank 1 and Bank 2 waveforms.
- Monitor desired and actual camshaft angles.
- Command the VVT system when supported.
- Warm the engine fully.
- Recreate the freeze-frame conditions.
- Complete the required drive cycle.
- Perform any manufacturer-specified relearn.
- Check for pending codes.
The repair is confirmed when the Sensor B waveform remains stable, actual camshaft position follows commands correctly and P0391 does not return.
Frequently Asked Questions
What does the P0391 code mean?
P0391 means the Bank 2 Camshaft Position Sensor B signal is present but outside the PCM’s expected range or performance criteria.
Is Sensor B the exhaust camshaft sensor?
Sensor B commonly monitors the exhaust camshaft on a DOHC engine, but service information must confirm the manufacturer’s designation.
Which side is Bank 2?
Bank 2 is the cylinder bank that does not contain cylinder number one. Its physical side varies by engine.
Does P0391 mean the camshaft sensor is bad?
Not necessarily. The sensor may be weak, but wiring resistance, air-gap problems, reluctor damage, VVT faults and mechanical timing can produce the same code.
What is the difference between P0390 and P0391?
P0390 identifies a general circuit malfunction or unusable signal. P0391 indicates that a signal exists but its waveform or response is implausible.
What is the difference between P0366 and P0391?
P0366 applies to Sensor B on Bank 1. P0391 applies to Sensor B on Bank 2.
What is the difference between P0019 and P0391?
P0019 specifically identifies crankshaft-to-camshaft correlation on Bank 2 Sensor B. P0391 identifies broader Sensor B signal or performance problems.
Can dirty oil cause P0391?
Yes. Dirty, low or incorrect oil can affect VVT solenoid and camshaft-phaser operation, causing actual camshaft response to fall outside the expected range.
Can a damaged reluctor cause P0391?
Yes. Bent, missing or shifted trigger features can distort waveform timing and pulse spacing.
How do you diagnose P0391?
Inspect the sensor circuit, capture its waveform, compare Bank 1 and Bank 2, and monitor commanded versus actual Bank 2 Sensor B camshaft position.
Final Thoughts
P0391 means the Bank 2 Camshaft Position Sensor B signal exists but does not perform as the PCM expects. That makes waveform analysis and camshaft-response data more valuable than simply checking whether the sensor has power.
Begin with oil condition, sensor identification and connector inspection. Then compare the Bank 2 Sensor B waveform with Bank 1 and the crankshaft signal. If the electrical waveform is clean, command the VVT system and compare desired versus actual camshaft position.
That process separates a weak sensor or damaged circuit from a reluctor, phaser or mechanical-timing problem without turning one range/performance code into a shopping list.
Visit the OBD-II Doctor for additional diagnostic guides or browse the complete P-series OBD-II code reference to identify related powertrain faults.
