AdBlue / SCRMercedes-Benz

Mercedes OM654 AdBlue System

OM654 AdBlue system faults can create countdown warnings and expensive diagnosis, so buyers should check SCR readiness, NOx data, and repair history.

Repair difficulty
Medium
Estimated cost
Medium to high depending on sensor/tank/module involvement.

Affected models and engines

  • Modern Mercedes diesel models using OM654 engines and SCR/AdBlue emissions systems

Symptoms

  • AdBlue countdown
  • Check-engine light
  • No-start warning
  • NOx efficiency faults

Why it fails

  • NOx sensors, dosing units, tank heaters, and software all interact.
  • Short trips and poor battery health can complicate diagnosis.
  • Cleared codes can return after a full drive cycle.

Checks before buying

  • Scan SCR and NOx modules with Mercedes-capable diagnostics.
  • Check for countdown history and recent code clearing.
  • Verify AdBlue tank, injector, and NOx sensor invoices.
  • Drive long enough for readiness checks.

Fault codes to investigate

P204FP20EEP2002

Fault codes guide diagnosis; they do not confirm a part without live data, inspection, service history, and repeat testing.

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P204F fault code

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This is buyer guidance, not a remote mechanical inspection or a guarantee. Always verify the car in person with diagnostics and service records.