When the SCR reservoir has been depleted for more than 10 hours and engine power derates, what is the correct service action?

• A. Fill the DEF reservoir and clear fault codes to return to service
• B. Fill the DEF reservoir and prime the DEF lines to remove the derate
• C. Replace the DEF reservoir
• D. Do nothing

Answer: (A)

When the SCR system runs out of DEF for an extended period, the engine control system shifts into a derate to protect emissions control. Restoring normal operation means restoring the DEF supply and telling the ECM that the fault is resolved. Filling the DEF reservoir supplies the urea solution the SCR catalyst needs, and clearing the fault codes lets the ECU exit derate once it confirms the DEF level is restored and there are no other active faults. Priming the DEF lines isn’t typically required in this situation, and replacing the reservoir isn’t necessary unless there’s a defect. Doing nothing would keep the engine in the derated state.