NOx sensors measure nitrogen oxides in the exhaust so the SCR system can dose diesel exhaust fluid correctly, and a failed one commonly triggers derates on modern diesels.
What it does
Modern diesels use upstream and downstream NOx sensors to monitor how well the SCR catalyst is reducing emissions. The computer uses their readings to control DEF dosing and to verify efficiency. A failed sensor gives bad data and often sets SCR faults.
Signs it is failing
- SCR or NOx sensor fault codes
- Countdown to a power derate
- Incorrect DEF dosing
- Reduced performance in severe cases
Replacement and what to know
A failed NOx sensor is replaced, and its wiring inspected. Because a bad sensor can falsely report low SCR efficiency, replacing a faulty sensor often clears catalyst-efficiency codes without touching the expensive catalyst.
Frequently asked questions
What does a NOx sensor do?
It measures nitrogen oxides in the exhaust so the SCR system can dose DEF correctly and verify catalyst efficiency.
Can a bad NOx sensor cause a derate?
Yes. NOx sensor faults commonly trigger a countdown to reduced power on SCR-equipped diesels.
Sensor or catalyst?
A faulty NOx sensor can falsely report low efficiency, so replacing it often clears codes without touching the catalyst.
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