GMC Code P1391: CKP/CMP Synchronization Problem
(Intermittent Loss of CMP or CKP Signal)
Technical Meaning
- Fault: The control module (PCM) detects an intermittent loss of synchronization between:
- CKP (Crankshaft Position Sensor): Crankshaft position sensor.
- CMP (Camshaft Position Sensor): Camshaft position sensor.
- Impact: Engine timing error → Desynchronization of injection and ignition → Risk of stalling, misfires, or hard starting.
Affected GMC Models
| Model | Common Engines | Years |
|---|---|---|
| GMC Sierra | 4.3L V6, 5.3L/6.0L V8 | 1999-2019 |
| GMC Yukon | 5.3L/6.2L Vortec | 2000-2020 |
| GMC Canyon | 2.8L/3.5L/3.7L I4/I5 | 2004-2012 |
| GMC Acadia | 3.6L V6 (LLT/LFX) | 2007-2017 |
| GMC Envoy | 4.2L I6, 5.3L V8 | 2002-2009 |
Common Symptoms
- ⚠️ Engine stalling at idle (especially when the air conditioning is active).
- 🔧 Hard starting (engine cranks for a long time before starting).
- ⚡ Misfires under acceleration or under load.
- 📈 Unstable idle (perceptible vibrations in the cabin).
- 💡 Check engine light on (sometimes accompanied by P0016 for desynchronization).
Specific Causes in GMC
- Electrical Problems (65-75% of cases):
- Damaged CKP/CMP wiring due to heat from the exhaust manifold (common on Sierra/Yukon).
- Oxidized connectors (moisture/road salt, especially on Canyon/Acadia).
- Blown fuse (check the under-hood fuse box: *PCM Fuse 10A/15A*).
- Mechanical Failures:
- Cracked or rusty crankshaft reluctor wheel (exposed Vortec V8 engines).
- Stretched timing chain (progressive symptom on high-mileage engines).
- Metal debris stuck to the CKP sensor (magnetism from wear particles).
- Other Causes:
- Oil leaks submerging the CMP sensor (aging seals on Envoy I6).
- Failing PCM (rare, but documented on Acadia 2007-2010).
Cost Analysis: The estimated cost of repairs related to GMC Code P1391 can vary significantly. Depending on the severity of the problem, costs can range from simple sensor replacements to more complex repairs, such as timing chains. It is advisable to get multiple quotes to ensure quality service at a reasonable price.
Step-by-Step Diagnosis
- Advanced OBD2 Scan:
- Check for related codes (P0335 = CKP, P0340 = CMP, P0016 = desynchronization).
- Observe live data: CKP:CMP Ratio must be 2:1 (2 crankshaft revolutions = 1 camshaft revolution).
- Physical Inspection:
- Sensor locations:
CKP: On the engine block, near the flywheel (access from underneath).
CMP: On the cylinder head, timing side (often hidden under intake manifolds). - Critical points:
- Wires rubbing against the exhaust manifold (Sierra 5.3L).
- CMP connector flooded with oil (Yukon 6.0L).
- Sensor locations:
- Electrical Tests:
Sensor Voltage (V) Resistance (Ω) CKP 5V or 12V 500–1,500 CMP 5V 200–1,000 🔍 Measure off-circuit with a multimeter.
Recommended Repairs
| Solution | Estimated Cost (USD) | Urgency |
|---|---|---|
| Replace CMP sensor | $60–120 | 🔧🔧 |
| Repair CKP harness | $80–200 | 🔧🔧 |
| Clean CKP reluctor wheel | $0 (DIY) | 🔧 |
| Replace timing chain | $800–1,500 | ⚠️⚠️ |
| Reprogram PCM | $150–300 | ⚠️ |
Typical GMC Case: On the Sierra 5.3L Vortec, the CKP harness runs near the exhaust manifold → Melted insulation = short circuit. Solution: Replace harness + heat shield.
Mistakes to Absolutely Avoid
- 🚫 Ignoring the timing chain: A stretched chain is a recurring cause on engines >150,000 km.
- 🚫 Forgetting to check fuses: A blown PCM fuse mimics a sensor fault.
- 🚫 Neglecting CMP seal: A leaking seal floods the sensor → Recurring failure.
Specific GMC Technical Tips
- Diagnostic Tools: Use a Tech2 or MDI scanner (official GM tools) to access advanced PCM parameters.
- Relearning Procedure: After replacing a sensor, perform a “Crank Position Sensor Variation Learn” via the scanner.
- Emergency Solution: If the engine stalls, restart it immediately – the PCM attempts an automatic resynchronization.
⚠️ Safety Warning:
- A P1391 code + severe misfires can damage the catalytic converter (overheating).
- If the check engine light flashes, stop the vehicle → Risk of catalytic converter destruction.
*Source: GMC Service Manual, technical service bulletins TSB 18-NA-355 (2018).*
ℹ️ Note: On Ecotec 3.6L engines (Acadia/Terrain), the P1391 code is often related to a faulty PCM control relay – check the under-hood relay box before any sensor replacement.
Additional Explanation: Understanding OBD II trouble codes is essential for diagnosing engine problems. When encountering a code like GMC Code P1391, it is imperative to examine other engine-related systems, as a failure in one component can often cause errors in others. For example, a crankshaft position sensor failure can affect the ignition system, leading to decreased performance.
Case Study: Consider a GMC Yukon owner who reported hard starting. After diagnosis, the GMC Code P1391 was found. By replacing the CMP sensor and checking the connections, the problem was resolved. This case illustrates the importance of a complete diagnosis before undertaking costly repairs.
Recommended Practices: To avoid future problems related to GMC Code P1391, it is advisable to perform regular vehicle inspections, particularly of cables and sensors. Preventive maintenance can reduce the risk of failure and improve your engine’s longevity.
Owner Testimonials: Many GMC owners who encountered GMC Code P1391 report significant improvements after proper repairs. These testimonials highlight the importance of prompt and appropriate interventions to avoid long-term damage, such as that caused to a catalytic converter.
Driver Education: Drivers should be familiar with the warning signals their vehicle may emit. A check engine light or unusual noise can be an indicator that something is wrong. By knowing the symptoms related to GMC Code P1391, owners can act quickly to avoid more costly repairs.