Explanation of Code P104C: “Reductant Level Sensor (AdBlue) Circuit – Voltage Too High”
Severity
- Level: 🟠 Moderate (risk of reduced anti-pollution performance and failing technical inspection).
- Urgency:
- Reduced power activated (limp mode).
- Exceeding emission limits (high NOx).
Common Symptoms
- 🚨 Check engine light on (MIL) and/or flashing SCR/AdBlue light.
- 🐌 Reduced power (“Limited starts remaining” message).
- 💡 Dashboard warning (e.g., “Low AdBlue level” even when tank is full).
Main Causes
- Faulty reductant level sensor:
- Sensor giving incorrect reading (phantom high voltage).
- Electrical issues:
- Short to ground in the sensor wiring harness.
- Connector corrosion (moisture or AdBlue leak).
- PCM failure (rare):
- Signal processing error or outdated software.
Diagnostic Procedure (Detailed Steps)
Step 1: Visual Inspection
- Locate the sensor: Integrated into the AdBlue tank (e.g., Mercedes Sprinter, Peugeot 508).
- Check for AdBlue leaks: Bluish corrosion around the connector.
Step 2: Sensor Electrical Test
- Multimeter:
- Power supply: 5V between the power terminal (red wire) and ground.
- Signal: 0.5-4.5V depending on AdBlue level (empty → 0.5V, full → 4.5V).
- If voltage > 4.8V → Short to ground or faulty sensor.
Step 3: PCM Circuit Check
- Continuity: Test wires between the sensor and PCM.
- Insulation resistance: No contact between signal and ground.
Solutions & Costs
| Problem | Solution | Average Cost |
|---|---|---|
| Faulty level sensor | Replacement | €150 – €400 |
| Wiring repair | Harness/connector repair | €50 – €200 |
| Connector cleaning | Contact spray + dielectric grease | €20 – €50 |
| PCM reprogramming | Software update | €100 – €300 |
Mistakes to Avoid
- ❌ Handling AdBlue without protection: Corrosive liquid (wear gloves/goggles).
- ❌ Ignoring AdBlue leaks: Can damage the sensor and tank.
Frequently Affected Models
- Mercedes-Benz Sprinter (2016-2022): Recurring sensor issues.
- Peugeot 508 (BlueHDi 180): Corroded connector.
- Volvo FH/FM (Euro 6): SCR system failures.
Practical Case
- Problem: Code P104C on a Mercedes Sprinter 2019 with “Insufficient AdBlue” message.
- Diagnosis: Level sensor stuck in high position (voltage at 5V).
- Solution: Sensor replacement (€320) + tank cleaning.
Pro Tips
- Reset the AdBlue system: After replacement, use a Star Diagnostic tool (Mercedes) or DiagBox (Peugeot).
- Use certified AdBlue: Avoid counterfeits to prevent deposits.
If the code persists after repair, consult a professional equipped with manufacturer diagnostic tools (e.g., Star Diagnostic or DiagBox)! 🔧🚗🔋