What's involved
What this repair actually is.
NOx sensors measure nitrogen-oxide levels before and after the SCR catalyst so the computer can dose DEF correctly, and they are one of the most common failures on modern diesels. We replace the failed sensor — upstream or downstream — with a new OE-quality unit, perform any required programming or relearn, and verify the SCR system passes its self-tests.
Symptoms
How you can tell.
- Check engine light with a NOx sensor code
- DEF warning or speed-limit countdown
- Poor fuel economy or extra DEF consumption
- Failed emissions inspection
If you delay
An ignored NOx fault starts the federal DEF countdown, ending in a 5-mph derate that will not clear until repaired.
Educational estimate. Real pricing depends on your vehicle, the inspection, parts availability, taxes, and shop rates. Use this as a guide while you shop quotes.