P0391 code definition
This is the generic OBD-II code for Camshaft Position Sensor "B" Circuit Range/Performance (Bank 2). Sensor B typically refers to the exhaust camshaft side, while Bank 2 indicates that the problem is on the side of the engine which does not contain cylinder #1.
What the P0391 Camshaft Position Sensor "B" Circuit Range/Performance (Bank 2)**
The camshaft position sensor tells the Powertrain Control Module (PCM) what position the camshaft is in, in order to facilitate proper fuel delivery and ignition timing. The sensor sends a signal to the PCM in the form of a voltage reading. If this reading deviates from the expected specifications, the computer detects that the camshaft position is abnormal, and Diagnostic Trouble Code (DTC) P0391 will set.
What causes the P0391 code?
Possible causes of this DTC include:
- A bad crankshaft position sensor
- An open or short in the sensor circuitry
- Bad connectors
- A bad starter motor
- Compromised battery function
- A fault in the starting system circuitry
What are the symptoms of the P0391 code?
Due to the fact that the camshaft position dictates fuel injection and cylinder firing timing, you may notice sluggish acceleration, poor engine performance, or rough engine idling. The vehicle may refuse to start at all, and the Check Engine or Service Engine Soon light may illuminate.
How does a mechanic diagnose the P0391 code?
Diagnosing P0391 may require:
Verifying any existing Diagnostic Trouble Codes using a scanner and recording relevant data, such as the order in which the codes occurred
Clearing all codes and road testing the vehicle above 45mph in order to see if the codes return
Diagnosing and repairing any errors which occurred prior to the camshaft sensor circuit problem
Visually inspecting the relevant wiring and connectors to check for corrosion, shorts, opens, or loose connections
Visually inspecting the circuitry for signs of engine oil or other fluids which may have spilled over, causing damage to the sensor or wiring
Testing the resistance and voltage of the camshaft position sensor and connector
Testing system continuity using a digital volt/ohmmeter
Keep in mind that some vehicles will set a camshaft sensor code when there is a faulty crankshaft sensor, so you may need to look into that possibility if none of the above diagnoses pan out.
Common mistakes when diagnosing the P0391 code
Frequent errors include automatically assuming the sensor is the problem, rather than examining and replacing any faulty wiring or connectors. In addition, if a circuitry problem is being cause by a leak, replacing the faulty parts won’t fix the issue. Other malfunctions must be addressed in order to prevent the code from returning.
How serious is the P0391 code?
This DTC should be taken seriously, as the camshaft position sensor has a direct effect on the engine’s function. Engine performance may be affected, resulting in driving conditions which are both unpleasant and unsafe. In addition, if the issue isn’t resolved promptly, other components could be affected and sustain damage.
What repairs can fix the P0391 code?
Fixing this DTC may involve:
- Repairing faulty wiring or connectors
- Replacing the starter motor
- Charging or replacing the battery
- Replacing the crankshaft position sensor
- Repairing the camshaft position sensor harness
- Repairing fluid leaks which may be damaging the sensor circuitry
Additional comments for consideration regarding the P0391 code
There are some cases in which aftermarket parts are sufficient, however experts recommend only using OEM parts when replacing sensors. Even though they may be more expensive, they experience failure much less frequently than non-OEM sensors, which saves you time, money, and hassle in the long run.
If you’re doing repairs to the camshaft sensor or related components, it’s best to replace both the camshaft and crankshaft versions at the same time, rather than individually. This ensures that they wear at the same rate, thus avoiding having to continually switch off replacing one or the other.
Need help with a P0391 code?
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