P0391

What it means

P0391 is the generic code for a range or performance fault in the circuit of camshaft position sensor B on bank 2. The engine control unit compares the camshaft signal with the crankshaft signal and has found the timing relationship or signal pattern is outside what it expects. The sensor is not necessarily dead, as the fault can come from the signal, the wiring or the mechanical timing. Which sensor is 'B' and which bank is bank 2 depends on the engine, so check the manufacturer data for the layout.

Symptoms

• Malfunction indicator lamp on
• Engine cranks for longer than usual before starting, or occasionally fails to start
• Rough idle or misfire
• Hesitation and loss of power
• Poor fuel economy
• Engine running in a reduced-power or limp mode on some models

Possible causes

• Faulty camshaft position sensor B (bank 2) giving a weak, erratic or distorted signallikely
• Damaged, chafed or corroded wiring, or poor connector contact in the sensor circuit (signal, supply or earth)likely
• Damaged, contaminated or incorrectly fitted reluctor or trigger wheel, or metal debris on the sensor tippossible
• Camshaft timing out of position due to a stretched or jumped timing chain or belt, or worn tensioner or guidespossible
• Fault in the variable valve timing system on bank 2, such as a sticking phaser or control valve, or low or dirty oilpossible
• Crankshaft position sensor fault or signal problem upsetting the comparisonpossible
• Fault in the engine control unit itselfrare

How to diagnose

1. Read all stored codes and freeze frame data, noting engine speed, temperature and load when the fault set, and look for related crankshaft, cam timing or misfire codes
2. Check the engine oil level and condition, as this affects variable valve timing operation
3. Visually inspect the bank 2 camshaft sensor, its connector and the wiring for damage, chafing, oil contamination, corrosion or looseness
4. Check that the sensor is fitted correctly and the mounting and air gap are right, and remove it to inspect the tip for debris and damage
5. Using the manufacturer's data, test the sensor supply, earth and signal circuits for continuity, shorts and voltage
6. View the camshaft and crankshaft signals together on an oscilloscope while cranking and running, and check the pattern and timing relationship
7. Compare the live camshaft timing values with the target, and test the variable valve timing control valve and phaser operation if fitted
8. If the signals suggest a mechanical fault, check the timing chain or belt position and wear, following the engine-specific procedure

How to fix

• Replace the bank 2 camshaft position sensor if it fails testing
• Repair or replace damaged wiring and connectors, and clean or reseat terminals
• Replace a damaged reluctor or trigger wheel and clear any metal debris
• Correct the valve timing, and replace the chain, belt, tensioner and guides as required
• Repair or replace faulty variable valve timing components, and change the oil and filter if they are contaminated or the oil is wrong
• Replace or repair the crankshaft sensor or control unit only when testing proves them faulty

Can I keep driving?

Nearby codes

• P0390GenericCamshaft Position Sensor B Circuit Bank 2
• P0392GenericCamshaft Position Sensor B Circuit Low Bank 2