System: Communication network (CAN bus) or internal module failure.
Key difference vs P138x:
- ➡️ P138x = Electrical/performance fault in a glow plug circuit.
- ➡️ P1390 = Failure of digital communication between the engine control unit (ECU) and the glow plug control module (GPCM).
Common Symptoms
- 🚫 Failure to start at temperatures < 5°C (even with glow plug indicator active),
- 💻 Check Engine light + error message (“Glow plug system fault”),
- 🔌 Complete absence of glow plug function (no relay click when ignition is on),
- ⚠️ Associated fault codes (U0100, U0423, etc.) related to the CAN network.
Priority Causes (check in this order)
- 📡 CAN bus communication problem:
- CAN-H / CAN-L wires cut, crushed, or short-circuited,
- Oxidized connectors (GPCM or ECU).
- 🔌 Defective GPCM power supply/ground:
- GPCM dedicated fuse blown (typically 10-15A),
- Corroded ground (check chassis ground point).
- 💻 Internal GPCM failure:
- Module no longer responding to ECU requests,
- Damaged electronic components (swollen capacitors, burnt circuits).
- 🖥️ ECU failure:
- Software or hardware failure of the control unit (rare).
Step-by-Step Diagnosis
1. Basic Electrical Checks:
| Element | Test |
|---|---|
| GPCM Fuse | Visual check + continuity |
| GPCM Power Supply | Measure voltage at power pin (ignition ON): 12V |
| GPCM Ground | Resistance between GPCM ground → Chassis: < 0.5 Ω |
2. CAN Network Test (digital multimeter required):
| Measurement | Normal Value | Anomaly |
|---|---|---|
| CAN-H Voltage | 2.5 – 2.7V (ignition ON) | < 1V = Short circuit |
| CAN-L Voltage | 2.3 – 2.5V (ignition ON) | > 4V = ECU problem |
| CAN Resistance | 60 Ω (between CAN-H and CAN-L) | 0 Ω = Short circuit / 120 Ω = Cut wire |
3. Advanced Diagnostic Scan:
- Use a diagnostic tool capable of reading the GPCM (e.g., Delphi, Autel MaxiCOM),
- Check:
- GPCM communication status (module “Not detected”),
- U0100 codes (loss of communication with GPCM),
- Live data (GPCM output current/voltage = 0).
Recommended Repairs
| Failure | Solution | Criticality |
|---|---|---|
| Blown fuse/faulty ground | Replace fuse / clean ground | 🔴 Urgent |
| Damaged CAN wires | Repair faulty section + shielding | 🔴🔴 High |
| Faulty GPCM | Replace + mandatory reprogramming* | 🔴🔴 High |
| Oxidized connectors | Clean contacts with compressed air + contact spray | 🟠 Medium |
* The new GPCM requires pairing with the ECU (professional tool essential).
Mistakes to Avoid
- ❌ Changing the GPCM without checking the CAN network → Recurring failure,
- ❌ Ignoring associated codes (e.g., U0100) → Incomplete diagnosis,
- ❌ Forcing startup → Risk of damaging the catalyst/DPF.
Practical Case (real PSA HDI example)
- Symptom: Failure to start when cold + P1390 code.
- Diagnosis:
- GPCM fuse OK,
- CAN-H Voltage = 0V (short circuit to ground),
- Trace: CAN wires crushed under ECU housing.
- Repair:
- Repair wires + insulate with heat shrink tubing.
Pro Tips
- 🔧 Quick test: Disconnect the GPCM connector → If the code changes to P1391 (open circuit), the GPCM is likely faulty.
- ⚡ Check history: A voltage spike (e.g., from incorrect jump start) can burn out the GPCM.
- 💡 Temporary solution:
- Short-circuit the glow plug relay (pins 30-87) to power all glow plugs → Allows emergency start (avoid long-term use).
⚠️ Important: This code often immobilizes the vehicle in cold weather. Prompt attention is crucial. If basic checks fail, consult a specialist equipped with a manufacturer tool (e.g., Diagbox for PSA, SD Connect for Ford).
Technical data: SAE J1939 standards, ISO 15765-2, Manufacturer WSMs.