Code Meaning
P20F4 indicates that the reductant (AdBlue®) flow is lower than expected by the SCR (Selective Catalytic Reduction) system. This fluid is essential for reducing NOx emissions on diesel vehicles.
Probable Causes
| Priority | Cause |
|---|---|
| 1. | Low AdBlue® level or degraded quality |
| 2. | Clogged cap/piping (urea crystallization) |
| 3. | Faulty AdBlue® pump |
| 4. | Clogged or faulty SCR injector |
| 5. | Defective AdBlue® quality/flow sensor |
| 6. | Electrical issue (wires, connectors, relay) |
| 7. | SCR module (DCU) failure |
Diagnostic Procedure
Step 1: Initial checks (40% of cases)
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✅ AdBlue® level:
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Check the tank (refill if < ⅓ full).
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Check fluid quality (no water or contaminants).
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🔍 Visual inspection:
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Examine pump/injector connectors and cables (look for corrosion, bent pins).
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Inspect AdBlue® lines (swelling, leaks, white crystals).
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Step 2: System tests (requires diagnostic tool)
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⚙️ Live data reading:
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Actual vs. expected AdBlue® flow rate.
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Tank temperature (must be > -7°C to operate).
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🔧 Pump/injector activation:
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Activate the pump with the diagnostic tool to verify its operation.
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Step 3: Electrical tests
| Component | Test |
|---|---|
| AdBlue® pump | Measure voltage (12V) and resistance (10-20 Ω depending on model) |
| SCR injector | Check resistance (often 10-15 Ω) |
| Level sensor | Verify voltage curve according to level |
Step 4: System cleaning
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� Decrystallization:
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Remove the SCR injector and soak it in hot distilled water (30 min).
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Use a specific cleaning kit for the lines (e.g., Liqui Moly SCR System Clean).
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Repair Solutions
| Cause | Solution | Estimated Cost |
|---|---|---|
| Low level | Refill with ISO 22241 certified AdBlue® | 10-20 € |
| Clogged lines | Chemical cleaning or replacement | 50-150 € |
| Faulty pump | Replacement | 300-600 € |
| Faulty SCR injector | Replacement | 200-500 € |
| Electrical issue | Wiring harness/connector repair | 50-200 € |
⚠️ Important:
Reset the SCR system with a diagnostic tool after repair (e.g., Delphi DS150E or Autel MaxiCOM).
An unresolved P20F4 can lead to power limitation and exceeding anti-pollution standards.
Technical Tips
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Crystal prevention:
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Use only new and certified AdBlue®.
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Avoid parking the vehicle on a slope (the injector must remain submerged).
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Related codes:
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If P20F4 is accompanied by P207F (flow too high), suspect a faulty sensor.
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Software update:
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Check your manufacturer’s TSBs (Technical Service Bulletins) for software corrections.
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