What this code means

The selective catalytic reduction system is not converting NOx emissions efficiently. The downstream NOx sensor shows higher than acceptable NOx levels.

Symptoms

• Check Engine Light
• DEF warning light
• Possible speed limitation countdown
• Increased NOx emissions
• Failed emissions test
• Derate mode if not addressed

Common causes

• Degraded SCR catalyst
• Incorrect DEF quality
• DEF dosing system malfunction
• Exhaust temperature too low for SCR
• SCR catalyst contaminated by sulfur

Diagnostic steps

1. Read downstream NOx sensor values
2. Check DEF quality with refractometer
3. Monitor SCR inlet and outlet temperature
4. Test DEF dosing quantity
5. Inspect SCR catalyst for contamination

Repair notes

Replace SCR catalyst if efficiency is permanently reduced. Verify DEF quality and dosing system. Check for fuel sulfur contamination.

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