The P1390 code on GMC vehicles (Sierra, Yukon, Canyon, Acadia) indicates a critical misalignment between the crankshaft (CKP) and camshaft (CMP), compromising engine control. Common on Vortec V8 and LLT V6 engines, this fault requires immediate attention to prevent engine destruction.
🚨 Immediate Symptoms
- Failure to start or sudden engine stalling.
- Severe jerking during acceleration + loss of power.
- Flashing “Check Engine” light (related codes: P0016, P0340).
- Metallic rattling near the timing cover (loose chain).
🔧 Specific GMC Causes
- Stretched timing chain (very common on 5.3L/6.2L V8 engines with over 100,000 km).
- Faulty hydraulic tensioner (internal leak or broken spring).
- Defective CKP/CMP sensor (flooded by oil leaks).
- Cracked or dirty reluctor wheel (missing teeth, magnetic debris).
- Insufficient oil pressure (failed pump, clogged filter, degraded oil).
🔍 Step-by-Step Diagnosis
- Check oil pressure:
- Minimum: 12 psi at idle, 40+ psi at 2,000 RPM (using ACDelco T-80 gauge).
- Inspect the timing chain:
- Access via side cover (on Sierra 1500, Yukon XL models).
- Check timing mark alignment (misalignment > 6° = danger).
- Test the sensors:
- CKP: Resistance 850–1350 Ω (V8 5.3L).
- CMP: AC 0.5–3V signal on oscilloscope.
- Advanced GM scan:
- Use Tech2/GDS2 to analyze CKP/CMP misalignment (> 2° = confirmed fault).
🛠️ Recommended Repairs (With ACDelco/GM References)
| Faulty Component | Solution | OEM Parts |
|---|---|---|
| Timing chain | Replace complete kit (chain + tensioners + guides) | ACDelco 12640486 (5.3L V8) |
| Hydraulic tensioner | Replace + check oil circuit | GM 12633825 |
| CKP sensor | Replace (air gap: 0.5–1.5 mm) | ACDelco 213-1151 |
| CMP sensor | Clean or replace | Delphi FG1105 (V6 3.6L) |
| Reluctor wheel | Replace crankshaft reluctor ring | GM 12586459 |
⚠️ Critical Warning:
Ecotec and Vortec engines are interference engines! Desynchronization causes piston-to-valve contact. Stop the engine immediately!
💡 Key Post-Repair Procedures
- Timing alignment:
- GM tools: Crankshaft lock tool J-42386 + Camshaft lock tool J-45027.
- ECU reset:
- Clear codes via Tech2 + sensor relearn procedure.
- Oil maintenance:
- 5W-30 Dexos1 (e.g., Mobil 1 ESP) + GM filter PF64.
📊 Technical Sheet
| Parameter | Value |
|---|---|
| OBD2 Code | P1390 |
| Definition | “Crankshaft/Camshaft Position Sensor Correlation Error” |
| Sensitive Models | Sierra, Yukon, Acadia, Canyon (2007–2022) |
| Urgency | MAXIMUM (risk of engine failure) |
Recurring Problems by Model
| Model | Common Failure | Solution |
|---|---|---|
| Sierra 1500 5.3L | Stretched chain (cold start rattling) | ACDelco kit 12640486 |
| Yukon 6.2L V8 | Seized hydraulic tensioner | Replace GM 12633825 |
| Acadia 3.6L V6 | Dirty CMP sensor (camshaft seal leak) | Delphi FG1105 + seal 12688049 |
Exclusive GMC Technical Tips
- Software update:
- On Sierra 2014–2018, an ECU reflash (TSB 18-NA-355) sometimes fixes false P1390 codes.
- Check oil filter:
- Non-OEM filters (PF64 required) clog tensioner oil passages.
- Leak inspection:
- A leaking rocker cover gasket floods CMP sensors (GM seal 12688049).
Conclusion
The P1390 code on GMC indicates a critical engine desynchronization. Proceed in 3 steps:
- Stop the engine and check oil/pressure (risk of immediate failure).
- Inspect the timing system and CKP/CMP sensors (80% of cases).
- Replace faulty components with ACDelco/GM parts and recalibrate with Tech2.
🔧 Average repair cost:
- Timing chain: $800–$2,000
- Sensors + labor: $300–$600
Do not drive – have it towed to a certified GM technician. Ignoring it can cost $5,000+ (engine replacement).