NOx Sensor

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📖 YOUCANIC Automotive Glossary

The NOx sensor measures nitrogen oxide concentration in diesel exhaust for SCR system operation. Vehicles typically have two: upstream (before SCR catalyst) tells the ECU how much NOx to calculate DEF injection rate, downstream (after catalyst) verifies SCR conversion effectiveness. The ECU compares levels to calculate efficiency — if downstream NOx exceeds legal thresholds, DTCs and power restrictions activate.

NOx sensor failures cause P2200-P2204 (circuit codes) and P229F/P20EE (efficiency below threshold). Failed sensors can trigger limp mode, DEF warnings, power restriction countdowns, or prevent restart. Sensors are expensive ($200-500+) and should be diagnosed thoroughly before replacement. After replacement, perform a NOx Reset with the YOUCANIC UCAN-II to clear adaptation values and reset efficiency counters.

Synonyms:
Nitrogen Oxide Sensor, NOx
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