Typical repair cost: $50–$4,500
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Most common cause. Use only API-certified DEF; refill before light comes on.
Upstream/downstream NOx sensors fail and trigger SCR efficiency codes.
No DEF dosing to the SCR catalyst.
Frozen DEF expands and cracks lines or pump.
Long-term low-RPM operation lets DEF crystallize and clog.
Derate happens fast. Get to a diesel shop before the 5-mph limit hits — you cannot drive home from that.
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