Mercedes-Benz C-Class: OM651/OM654 AdBlue SCR NOx Sensor Failure and Engine Restart Lockout

Both the OM651 and OM654 Euro 6 diesel engines use a Selective Catalytic Reduction (SCR) system with AdBlue urea dosing and upstream and downstream NOx sensors. The NOx sensors degrade with heat and mileage, providing drifting readings that cause the ECU to lose confidence in SCR efficiency and trigger a countdown warning mode. If the fault is not resolved, the engine management system will prevent the engine from restarting after a fixed number of starts, regardless of the AdBlue tank level.

• AdBlue warning light or message to refill despite the tank being adequately filled
• Check engine light alongside SCR emissions system warnings
• Instrument cluster message counting down kilometres until engine restart is prevented
• Limp mode or reduced engine power following an emissions system fault
• P2200 or P2201 NOx sensor fault codes on diagnostic scan

1. 1

   Check the AdBlue tank level and ask whether any countdown or AdBlue warning messages have appeared on the instrument cluster.

2. 2

   Inspect the AdBlue dosing pump area for white crystalline deposits, which indicate a leaking or stuck-open injector.

3. 3

   Verify only ISO 22241-compliant AdBlue fluid has ever been used.

4. 4

   If a workshop inspection is possible: Before purchase, have a workshop perform a full diagnostic scan specifically checking for P2200, P2201, P220A, or P220B NOx sensor codes and any SCR efficiency fault codes.

≈A$740 to ≈A$4,600 at independent workshop rates, shown as an approximate guide in Australian dollars. That is for this job on its own, not for the car.

An estimate, converted to Australian dollars, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has the car ever shown an AdBlue warning, countdown message, or engine restart prevention? What was done to resolve it?”
• “Have any NOx sensors been replaced? Which ones and at what mileage?”
• “Has only approved ISO 22241 AdBlue fluid been used, and when was the SCR system last inspected?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.