Code P1391 (sometimes listed as P1391 00 in modern OBD-II systems) is an error code related to engine timing. It signals a correlation problem between the crankshaft position sensor (CKP) and the camshaft position sensor (CMP). This fault can lead to critical malfunctions such as stalling, misfires, or hard starting.
Technical Meaning
- OBD-II Definition:
“Crankshaft Position Sensor / Camshaft Position Sensor – Correlation”.
The engine control unit (ECU) detects an abnormal discrepancy between the signals from the two sensors, which are essential for controlling injection and ignition. - Consequences:
Misalignment disrupts valve timing and injection, risking engine damage (e.g., piston/valve collision in interference engines).
Possible Causes
- Mechanical Issues:
- Loose, worn, or improperly timed timing belt/chain.
- Faulty CKP/CMP alignment sprocket.
- Slippage of the camshaft variator (in VVT systems).
- Electrical Failures:
- Faulty CKP or CMP sensor (cut wires, oxidized connector).
- Damaged or dirty reluctor wheel (magnetic target).
- Other Causes:
- Poor quality engine oil (affects VVT systems).
- Voltage issue (weak battery, faulty alternator).
Common Symptoms
- 🚨 Check Engine Light (MIL) illuminated.
- 🔧 Misfires and power loss.
- 💨 Overheating or abnormal exhaust smoke.
- 🔋 Hard starting (engine cranks but won’t start).
- ⚠️ Stalling at low RPM or during acceleration.
Diagnostic Procedure
- Code Reading:
Use an OBD-II scanner to check for associated codes (e.g., P0016/P0017). - Visual Inspection:
- Check the timing belt/chain (tension, alignment).
- Inspect sensor cables and connectors.
- Electrical Tests:
- Measure CKP/CMP sensor resistance (typical value: 500–1500 Ω).
- Analyze the signal with an oscilloscope (sine or square wave).
- Mechanical Check:
- Verify camshaft alignment with a dedicated locking tool.
- Examine the crankshaft reluctor wheel (cracks, dirt).
Repair Solutions
- Sensor Replacement (CKP/CMP): Average cost €80–150 (part).
- Timing Realignment:
- Replacement of timing belt/chain + timing kit: €400–900.
- Verification of sprocket alignment.
- Reluctor Wheel Cleaning/Replacement: If magnetic and dirty.
- ECU Update: If software error (rare cases).
⚠️ Important: A P1391 error requires professional diagnosis. Attempting repair without expertise may worsen damage (e.g., engine failure).
Case Studies
- Volkswagen Golf 1.9 TDI (2005): P1391 code caused by a cracked reluctor wheel on the crankshaft → Flywheel replacement.
- Toyota Corolla VVT-i (2012): Timing drift due to a stuck camshaft variator → VVT system cleaning + appropriate engine oil.
FAQ
Q1: *Can I drive with a P1391 code?*
→ Not recommended. Risk of stalling or engine damage.
Q2: *Does a faulty sensor always trigger P1391?*
→ Yes, but check the timing first (most common cause).
Q3: Average repair cost?
→ €200–1000 depending on the cause (sensors vs. timing).
Conclusion
Code P1391 is a serious alert regarding engine timing integrity. Although sometimes related to simple sensors, it often hides critical mechanical issues (timing). Precise diagnosis by a professional is essential to avoid costly repairs. If this code appears, stop the vehicle and consult a garage immediately.