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July 23, 2025 Service bulletin

Check engine light with fault code P300E3B in the CDI control unit CDI (N3/28) Complaint Check engine...

Mercedes-Benz technical service bulletin LI49.20-N-076565 for 2021 Mercedes-Benz Sprinter 2500, 2021 Mercedes-Benz Sprinter 3500 and 2021 Freightliner Sprinter 2500 and others.

Bulletin number
LI49.20-N-076565
Component
Unknown or other
Summary
Check engine light with fault code P300E3B in the CDI control unit CDI (N3/28) Complaint Check engine light is illuminated with fault code P300E3B “The efficiency of component 'SCR catalytic converter' is not sufficient" stored in the CDI control unit (N3/28). Cause Excessive ammonia slip is being detected in the SCR system, often triggered by IUMPR (In-Use Monitor Performance Ratio) thresholds. This typically indicates that the SCR catalyst is not converting DEF (AdBlue) efficiently under moni- tored conditions. Common Driving Scenario: •Non-dynamic driving (e.g., highway cruising with cruise control set) •Minimal load changes, steady-state exhaust flow Contributing Factors: •Software influence – IUMPR (In-Use Monitor Performance Ratio) thresholds •DEF Overdosing – Over-injection due to faulty NOx sensors, dosing valve issues, or poor DEF quality •Low Exhaust or Catalyst Temperatures – Common under low-load conditions; reduces SCR conversion efficiency •Catalyst Degradation or Contamination – Aging catalyst or contamination from oil, coolant, or poor-quality DEF Remedy IMPORTANT: Do not replace any components unless instructed through TIPS. For Model Year 2019–2021 Vehicles: •Update the following control units to the latest available software: N3/28 – Motor Electronics 'CDI61NFZ' for combustion engine 'OM642' (CDI):

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