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

The P0391 diagnostic trouble code means the engine control module (ECM/PCM) has detected a problem with the range or performance of the Camshaft Position Sensor “B” circuit on Bank 2.

Unlike a circuit-high or circuit-low code, P0391 does not necessarily mean that the sensor signal is simply too high or too low. Instead, the ECM has determined that the signal is present but does not behave as expected.

The problem may involve the camshaft position sensor, wiring, connector, trigger wheel, variable valve timing system, or even the mechanical relationship between the camshaft and crankshaft.

Most importantly, P0391 does not automatically mean the camshaft position sensor needs to be replaced.

Understanding Sensor “B” and Bank 2

There are two terms in this code that need to be understood separately.

Bank 2 refers to the cylinder bank that does not contain cylinder number 1.

This designation is mainly relevant to engines with multiple cylinder banks, such as V6 and V8 engines. Bank 2 is not universally the left or right side of the engine.

Sensor “B” is a manufacturer-specific designation.

On some engines, Sensor B may correspond to the exhaust camshaft, while Sensor A may correspond to the intake camshaft. On other engines, the designation can follow a different strategy.

Therefore, the exact physical sensor represented by P0391 must be confirmed using the vehicle's wiring diagram or manufacturer service information.

What Does “Range/Performance” Mean?

The Range/Performance description is different from "High Input," "Low Input," or "Circuit Open."

With P0391, the ECM is generally receiving a camshaft signal, but the signal is not behaving within the expected operating parameters.

For example, the signal may indicate that the camshaft is moving, but its timing relative to the crankshaft is incorrect.

The ECM may also detect:

• Incorrect signal timing

• Unexpected signal frequency

• Irregular signal behavior

• Camshaft position outside the expected range

• Signal that does not correspond correctly to engine speed

• Incorrect response from a VVT-controlled camshaft

• Intermittent signal abnormalities

This means P0391 can involve both electrical and mechanical causes.

How the Camshaft Position Sensor Works

The camshaft position sensor monitors the position and rotation of the camshaft.

As the camshaft rotates, a reluctor wheel, trigger wheel, magnetic target, or other reference feature passes the sensor.

The sensor generates an electrical signal that the ECM uses to determine camshaft position.

The ECM then compares this information with crankshaft position.

This synchronization allows the engine computer to determine precisely where the engine is within its combustion cycle.

Camshaft information can be used for:

• Fuel injection timing

• Ignition timing

• Variable valve timing

• Cylinder synchronization

• Misfire detection

• Engine starting

• Camshaft actuator control

A problem with the signal can therefore affect several engine-management functions simultaneously.

Common Causes of P0391

P0391 has a relatively broad range of possible causes.

Faulty camshaft position sensor

The sensor may produce an unstable, weak, distorted, or otherwise incorrect signal.

A sensor can work correctly at idle but become unreliable when engine speed or temperature increases.

Heat-related sensor failures are particularly important because the problem may disappear after the engine cools down.

Damaged or contaminated sensor

Oil, metal particles, moisture, or other contamination can affect the sensor or the area around its sensing element.

A sensor installed incorrectly or positioned incorrectly can also produce an abnormal signal.

Damaged wiring

The wiring harness may have:

• Broken conductors

• Chafed insulation

• Heat damage

• Corrosion

• Poor previous repairs

• Loose terminals

A wiring fault may only occur when the engine moves or the harness vibrates.

Poor electrical connector connection

A connector that looks normal can still have poor terminal contact.

Possible problems include:

• Corroded pins

• Loose terminals

• Poor terminal tension

• Water intrusion

• Oil contamination

• Broken locking mechanism

Power or ground problem

Depending on the sensor design, an incorrect supply voltage or poor sensor ground can distort the signal.

If several sensors show unusual behavior simultaneously, a shared power, reference, or ground circuit should be investigated.

Camshaft trigger or reluctor problem

The sensor may be perfectly functional while the target it reads is damaged.

Possible problems include:

• Damaged reluctor teeth

• Missing trigger features

• Loose reluctor wheel

• Incorrect trigger component

• Excessive mechanical movement

• Contamination around the trigger mechanism

Variable valve timing problem

VVT systems can change camshaft position relative to the crankshaft.

A malfunctioning VVT actuator can cause the actual camshaft position to differ from what the ECM expects.

Possible causes include:

• Sticking VVT actuator

• Faulty oil control solenoid

• Restricted oil passages

• Incorrect oil viscosity

• Low oil pressure

• Dirty engine oil

• Mechanical wear

P0391 can therefore sometimes be associated with the VVT system rather than the sensor itself.

Low engine oil or oil-pressure problems

Variable valve timing systems depend on proper oil pressure and clean oil.

If oil pressure is insufficient, the camshaft actuator may not achieve the commanded position.

This can cause the ECM to detect a camshaft position performance problem.

Mechanical timing problem

A stretched timing chain, worn timing components, incorrectly installed timing belt, or jumped timing can alter the camshaft-to-crankshaft relationship.

This is an important consideration if P0391 appears together with cam/crank correlation codes.

Incorrect timing after engine repair

If the engine has recently undergone work involving the timing chain, timing belt, cylinder head, camshaft, or VVT system, incorrect mechanical timing should be investigated.

A small timing error can produce a camshaft performance fault even though the sensor itself works correctly.

ECM/PCM problem

A control-module fault is possible but relatively uncommon.

The ECM should normally be considered only after the sensor, wiring, power, ground, trigger mechanism, VVT system, and mechanical timing have been tested.

Symptoms of P0391

P0391 can produce anything from a Check Engine Light with no noticeable symptoms to significant engine-performance problems.

Possible symptoms include:

• Check Engine Light

• Difficult starting

• Extended cranking

• Intermittent no-start

• Rough idle

• Engine hesitation

• Poor acceleration

• Reduced engine power

• Engine misfires

• Increased fuel consumption

• Stalling

• Poor throttle response

• Variable valve timing problems

• Limp-home mode

The severity depends on how much the camshaft signal has deviated from the expected behavior.

Can P0391 Cause a No-Start?

Yes.

The camshaft signal is important for engine synchronization.

If the ECM cannot reliably determine the camshaft position, it may have difficulty synchronizing fuel injection and ignition.

Depending on the vehicle, the engine may:

• Crank longer than normal

• Start intermittently

• Start and immediately stall

• Crank without starting

Some engines can continue operating using a backup strategy, while others depend more heavily on camshaft information.

Can P0391 Cause Misfires?

Yes.

An incorrect camshaft signal can affect injection timing, ignition timing, or variable valve timing.

This can lead to incorrect combustion timing and potentially produce misfires.

However, a misfire does not prove that the camshaft sensor is the cause.

Ignition coils, spark plugs, injectors, fuel pressure, compression, valve timing, and other engine-management problems can also produce misfire symptoms.

P0391 and Variable Valve Timing

VVT deserves special attention when diagnosing P0391.

In a VVT-equipped engine, the ECM may command the camshaft to move to a specific position.

The camshaft position sensor then reports the actual position.

The ECM compares:

Desired camshaft position → Actual camshaft position

If the camshaft does not reach the requested position, or reaches it too slowly, the ECM may interpret this as a range/performance problem.

A faulty sensor is therefore only one possibility.

The real problem could be:

• VVT solenoid

• Oil pressure

• Dirty oil

• Restricted oil passage

• VVT actuator

• Mechanical timing

• Camshaft mechanism

This is why replacing the sensor without checking the VVT system can result in an unnecessary repair.

How P0391 Is Diagnosed

A good diagnosis starts by determining whether the problem is electrical, signal-related, VVT-related, or mechanical.

1. Scan all diagnostic codes

Do not diagnose P0391 alone.

Look for codes related to:

• Other camshaft sensors

• Crankshaft position

• Cam/crank correlation

• VVT

• Oil pressure

• Engine misfire

• Sensor reference voltage

• Sensor circuits

Additional codes can significantly narrow the diagnosis.

2. Check freeze-frame data

Look at the conditions under which P0391 was stored.

Useful information includes:

• Engine RPM

• Coolant temperature

• Engine load

• Vehicle speed

• Battery voltage

• Throttle position

• Camshaft position

• VVT commanded position

• VVT actual position

If the code occurs only during cold starts, hot operation, acceleration, or high RPM, this can provide an important clue.

3. Identify Sensor B on Bank 2

Confirm the exact sensor using manufacturer documentation.

Do not assume that Sensor B is always the exhaust camshaft.

The designation varies between engine families.

4. Inspect the sensor and connector

Check for:

• Oil contamination

• Moisture

• Corrosion

• Damaged terminals

• Loose pins

• Broken connector locks

• Physical sensor damage

Also inspect whether the sensor is correctly installed.

5. Inspect the wiring harness

Follow the harness from the sensor toward the ECM.

Look for:

• Burned insulation

• Chafing

• Broken wires

• Pinched wiring

• Poor repairs

• Contact with moving components

• Oil contamination

If the problem is intermittent, gently moving the harness while monitoring the signal can help reveal a wiring fault.

6. Verify power, reference, and ground

The correct test depends on the sensor design.

Some sensors use a power supply, ground, and digital signal.

Others use a different electrical arrangement.

Do not apply generic voltage or resistance values from another vehicle.

Use the manufacturer's wiring diagram and specifications.

7. Monitor live camshaft data

A scan tool can often display:

• Camshaft position

• Camshaft synchronization

• VVT commanded angle

• VVT actual angle

Watch these values during startup, idle, acceleration, and different engine temperatures.

A value that suddenly jumps or stops responding can provide a valuable clue.

8. Compare camshaft and crankshaft signals

If the sensor circuit appears correct, compare the camshaft signal with the crankshaft signal.

This is particularly important if P0391 is accompanied by a correlation code.

An abnormal relationship may indicate:

• Timing chain stretch

• Timing belt misalignment

• Jumped timing

• Damaged reluctor

• VVT malfunction

• Incorrect camshaft installation

9. Test the VVT system

If the engine uses variable valve timing, verify that the commanded camshaft position matches the actual position.

Check the VVT solenoid, oil supply, actuator, and relevant mechanical components.

Low oil level or incorrect oil condition should also be investigated.

10. Use an oscilloscope when necessary

An oscilloscope can provide a much more detailed picture of the camshaft sensor signal.

It can reveal:

• Missing pulses

• Irregular pulses

• Signal distortion

• Electrical noise

• Dropouts

• Incorrect amplitude

• Timing abnormalities

Comparing camshaft and crankshaft waveforms can be particularly valuable when diagnosing intermittent or difficult P0391 faults.

Why Sensor Replacement Does Not Always Fix P0391

A common mistake is to see a camshaft sensor code and immediately install a new sensor.

Suppose the camshaft sensor is producing a perfectly good signal but the timing chain has stretched.

The ECM will still see the camshaft position occurring at the wrong time.

Replacing the sensor will not correct the mechanical timing.

Similarly, if a VVT actuator is sticking because of contaminated oil, a new sensor will not solve the problem.

The same applies to damaged wiring or a defective reluctor.

P0391 identifies a performance problem involving the sensor signal, not necessarily a failed sensor.

P0391 vs P0392 and P0393

The three codes describe different conditions involving Camshaft Position Sensor “B,” Bank 2.

P0391 – Camshaft Position Sensor “B” Circuit Range/Performance

The signal is present but does not behave within the expected range or performance parameters.

P0392 – Camshaft Position Sensor “B” Circuit Low Input

The ECM detects an abnormally low electrical signal.

P0393 – Camshaft Position Sensor “B” Circuit High Input

The ECM detects an abnormally high electrical signal.

This distinction is useful because P0391 requires broader investigation. It can involve not only the electrical circuit but also the actual timing and behavior of the camshaft.

P0391 vs Cam/Crank Correlation Codes

A camshaft performance code and a cam/crank correlation code can be closely related, but they are not identical.

A correlation code generally focuses on whether the camshaft and crankshaft are correctly synchronized.

P0391 focuses on the expected range/performance of the Sensor B signal.

If both types of codes appear together, mechanical timing, VVT operation, and the camshaft trigger mechanism deserve particular attention.

Can You Drive With P0391?

If the Check Engine Light is steady and the engine runs normally, short-term driving may be possible.

However, P0391 should not be ignored for an extended period.

A camshaft signal problem can become worse and potentially result in:

• Stalling

• No-start condition

• Misfires

• Reduced power

• Poor fuel economy

If the engine is severely misfiring, repeatedly stalling, or experiencing major power loss, continued driving is not advisable.

A Practical Diagnostic Sequence

A useful diagnostic process is:

P0391 detected → scan all codes → review freeze-frame → identify Sensor B/Bank 2 → inspect sensor and connector → inspect wiring → verify power/reference and ground → monitor camshaft live data → compare commanded and actual VVT position → inspect camshaft signal waveform if necessary → compare camshaft and crankshaft synchronization → check mechanical timing → repair the confirmed fault → clear the code and perform a verification drive.

This sequence helps avoid replacing a sensor when the real problem is wiring, VVT, oil pressure, or mechanical timing.

Final Perspective

P0391 means the ECM/PCM has detected a range or performance problem involving Camshaft Position Sensor “B” on Bank 2.

The sensor itself can certainly be responsible, but the diagnosis should not stop there.

A proper investigation should determine whether the sensor is producing the correct signal, whether the wiring and electrical circuits are healthy, and whether the camshaft is actually moving to the position expected by the ECM.

If the electrical signal is correct but the camshaft is not reaching the commanded position, attention should move toward VVT operation, oil pressure, mechanical timing, and the camshaft trigger mechanism.

That is the key to P0391: the code identifies abnormal camshaft signal performance, not a guaranteed failed camshaft position sensor.