P22FA
NOx Sensor Performance - Slow Response High to Low (Bank 1 Sensor 1)
NOx sensor on bank 1 sensor 1 responds too slowly when NOx concentration falls (typically observed when SCR dosing increases). The sensor has aged or its protection cap is clogged.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Common causes
Ordered by how often each turns out to be the cause.
- Sensor agedCommon
- Sensor element aged beyond calibration windowLess likely
- Sensor element contaminated by oil, soot or coolantLess likely
Parts involved
- NOX Sensor
P22FA is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.