P206B
Reductant Quality Sensor Circuit Range/Performance
DEF quality sensor reads in range but reports an off-spec urea concentration — typically diluted DEF or non-DEF fluid in the tank.
- 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.
- DEF diluted with waterCommon
- Wrong fluid in DEF tankPossible
- Sensor element aged beyond calibration windowLess likely
Parts involved
- Reductant Quality Sensor
P206B 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.