System: Global Control Circuit (GPCM) or Common Electrical Fault.
Clear Difference vs P138A-E:
- ➡️ P138A to P138E = Problem on 1 specific cylinder.
- ➡️ P138F = Collective anomaly (power supply, ground, module, or multiple glow plugs).
Common Symptoms
- ❄️ Unable to start below 5°C (despite glow plug light),
- 💡 Engine light + glow plug light on simultaneously,
- 🌫️ Excessive white smoke from exhaust,
- 🔌 Repeated relay clicking during startup,
- ⚠️ Very unstable idle during first 2 minutes.
Priority Causes (check in this order!)
- ⚡ Faulty main power supply:
- Blown main fuse (typically 40A to 80A),
- Failed glow plug relay (stuck/burnt winding).
- 🔌 GPCM ground issue:
- Corroded/loose ground cable (check chassis connection).
- 💻 GPCM Module Failure:
- Internal short circuit, overheating, or software failure.
- 🔧 Multiple defective glow plugs:
- Simultaneous wear (especially if >200,000 km),
- Open or short circuit on multiple cylinders.
- 🧩 Damaged wiring harness:
- Rubbed/burned main wires (e.g., contact with exhaust).
Step-by-Step Diagnosis
1. Quick Electrical Checks:
| Component | Test | Expected Value |
|---|---|---|
| Main fuse | Visual + continuity | No break |
| Glow plug relay | Power the coil | Click + continuity 30-87 |
| GPCM ground | Measure resistance GPCM → Chassis | < 0.5 Ω |
| GPCM power | Voltage at power pin (ignition ON) | Stable 12V |
2. Glow Plug Test (collective):
- Disconnect main GPCM connector,
- Measure total resistance between power terminal and ground:
Normal resistance = (Glow plug R) / number_of_cylinders
Ex: 6 plugs of 1Ω → ≈ 0.16 Ω - If ∞ Ω: Open circuit (fuse, cut wires),
- If ≈ 0 Ω: General short circuit.
3. Advanced GPCM Scan:
- Use a diagnostic tool capable of communicating with GPCM (e.g., Delphi, Snap-on),
- Check live parameters:
- Input voltage,
- Current draw,
- Concurrent fault codes P138A to P138E.
Recommended Repairs
| Failure | Solution | Criticality |
|---|---|---|
| Blown fuse/relay | Replace + check overcurrent cause | 🔴 Urgent |
| Corroded ground | Clean + tighten connection | 🔴 Urgent |
| Faulty GPCM | Replace + mandatory reprogramming* | 🔴🔴 High |
| Multiple failed plugs | Replace complete set | 🟠 Medium |
| Damaged harness | Repair defective section | 🟢 Low |
* GPCM often requires pairing with ECU (professional tool required).
Common Mistakes to Avoid
- ❌ Changing GPCM without checking fuse/relay → New module burned out,
- ❌ Forgetting GPCM reprogramming → Persistent fault code,
- ❌ Replacing single glow plug on high-mileage vehicle → Recurring failure.
P138x Code Summary Table
| Code | Target | Priority Test |
|---|---|---|
| P138A | Cyl 1 | Plug 1 resistance |
| P138B | Cyl 2 | Cyl 2 wiring |
| P138C | Cyl 3 | Plug 3 |
| P138D | Cyl 4 | Cyl 4 power |
| P138E | Cyl 5 | GPCM ground |
| 🛑 P138F | System | Fuse + Relay + GPCM Ground |
Pro Tips
- 🔧 If P138F + other P138x codes: Fault always comes from GPCM or power supply.
- ⏱️ Check history: An old short circuit (e.g., melted plug) may have damaged relay/GPCM.
- 💡 Quick test: Temporarily short the relay (terminal 30 to 87):
- If engine starts → Relay faulty,
- If no change → Fuse/GPCM/wiring issue.
⚠️ Important: This code can immobilize vehicle in cold weather! Quick attention is crucial.
Definitive solution: Entrust diagnosis to specialist with manufacturer diagnostic tool (e.g., SD Connect for Ford/PSA, ODIS for VAG). Live PIDS data from GPCM is essential! 🔧