Common Symptoms
- ❄️ Hard starting in cold weather (engine cranks for a long time before starting),
- 🚗 Unstable idle or vibrations at startup,
- ⚡ Misfires (especially when cold),
- 💡 Engine fault light (Check Engine/ESP) illuminated,
- 📉 White smoke from exhaust (unburned fuel).
Possible Causes
- 🔧 Faulty glow plug (cylinder 4): Internal failure (80% of cases).
- 🔌 Electrical issue in cylinder 4 circuit:
- Cut/oxidized wires between GPCM and glow plug,
- Defective or corroded connector.
- 💻 Control Module (GPCM) failure: Failure of the circuit dedicated to cylinder 4.
- ⚡ Faulty power supply:
- Defective glow plug relay,
- Blown fuse (check fuse box).
Step-by-Step Diagnosis
- 📏 Test glow plug #4:
- Disconnect its connector,
- Measure its resistance with a multimeter:
- Normal value: 0.1 – 2 Ω (check manufacturer specification),
- Infinite resistance = dead glow plug.
- Alternative: Use a glow plug tester (red glow = OK).
- 🔍 Inspect wiring:
- Follow the wire from GPCM to cylinder 4,
- Look for burn marks, cuts, or corrosion.
- 🔋 Check power supply:
- Test voltage at glow plug terminals during startup (should be ~10-12V for 2-10 sec).
- 🧩 Check GPCM and relay:
- Scan the module for associated fault codes (e.g., P138A, P138B, P138C),
- Test the relay (audible click + continuity).
Recommended Repairs
| Faulty Component | Action | Average Cost |
|---|---|---|
| Glow plug #4 faulty | Replace glow plug | 20-80 € |
| Damaged wiring | Repair wires/connectors | 50-150 € (labor) |
| GPCM failure | Replace module | 150-400 € (part) |
| Relay/fuse faulty | Replace part | 10-40 € |
⚠️ Important:
- Never force start if a glow plug is faulty → risk of damaging the GPCM!
- Replace glow plugs in sets (if >100,000 km) to avoid repeated failures.
- Reset ECU after repair (using diagnostic tool).
Common Mistakes to Avoid
- Neglecting connector sealing (causing oxidation),
- Forgetting to calibrate GPCM after replacement (requires reprogramming).
ℹ️ Pro tip: If multiple P138x codes appear (e.g., P138C + P138D), the failure likely comes from the GPCM or relay, not the glow plugs!
If diagnosis exceeds your skills, consult a diesel mechanic 🔧. A specific diagnostic tool (e.g., Delphi, Snap-on) is often essential!
Sources: Manufacturer data (PSA, Renault, VAG), WSM, ETKA.