P21C6
Reductant Level Sensor B Stuck
DEF level sensor B reading does not change over a refill or consumption window — float arm jammed or sender wiper stuck. Distinct from a circuit fault because the electrical signal stays plausible.
- 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.
- Float arm jammedCommon
Parts involved
- Reductant Level Sensor
P21C6 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.