Key Symptoms
- 🥶 Very difficult starting (especially below 10°C),
- 🌫️ Thick white smoke from the exhaust (unburned fuel),
- 📳 Idle vibrations or intermittent stalling,
- ⚠️ Check Engine light on (+ possible glow plug light).
Probable Causes (by frequency)
- Faulty glow plug (cylinder 5):
- Resistance out of specification (normal value: 0.1 – 2 Ω),
- Internal short circuit or open circuit.
- Wiring problem (cylinder 5):
- Cut, chafed, or oxidized wires (especially near the exhaust manifold),
- Loose/corroded connector.
- GPCM Module Failure:
- Failure of the circuit dedicated to cylinder 5,
- Module overvoltage/overheating (often caused by a faulty glow plug).
- General power supply issue:
- Stuck/faulty glow plug relay,
- Blown fuse (check amperage ≥ 40A).
Step-by-Step Diagnosis
1. Test glow plug #5:
- Disconnect the connector,
- Measure the resistance between the + terminal and the glow plug ground: ∞ Ω (infinity) → Dead glow plug < 0.1 Ω → Short circuit > 2 Ω → Resistance too high
- Alternative: Use a glow plug tester (should glow red in 5 sec max).
2. Inspect wiring (priority!):
- Visually check the wire from GPCM to cylinder 5 (look for melted insulation, corrosion),
- Test continuity wire by wire (multimeter in Ω mode),
- Check the supply voltage: 10-12V during glow plug activation.
3. Check GPCM/Relay:
- Scan the module: associated codes (P138A to P138D) indicate GPCM failure,
- Test the relay:
- Audible click upon activation,
- Continuity between terminals 30-87 when powered.
Recommended Repairs
| Failure | Solution | Estimated Cost |
|---|---|---|
| Glow plug #5 faulty | Replace the glow plug | 25-90 € |
| Damaged wiring | Repair the harness | 60-200 € |
| Defective GPCM | Replace + reprogram* | 200-500 € |
| Faulty relay/fuse | Replace the part | 10-50 € |
* Some vehicles require GPCM pairing with the ECU (professional diagnostic tool required).
Common Mistakes to Avoid
- Replacing the glow plug without checking the wiring → New fault code within 48h,
- Forgetting to check the circuit ground (corrosion on chassis ground point),
- Forcing the start with a faulty glow plug → Accelerated GPCM deterioration.
Technical Tips
- 🔧 Replace in sets: If >150,000 km, change all glow plugs (avoids cascade failures).
- ⚡ GPCM Protection: Always disconnect the battery before working on the circuit.
- 💡 Reset: Clear codes after repair + perform a glow plug cycle (without starting) for verification.
⚠️ Warning: On some engines (e.g., BMW M57, Mercedes OM642), accessing cylinder 5 often requires removing parts (particulate filter, piping).
ℹ️ If the code reappears after replacement: Have the GPCM timing/calibration checked (parameter setting via diagnostic tool) or check for a compression leak at cylinder 5 (weakens the glow plug effect).
Consult a diesel specialist if the diagnosis is complex! 🔧