P21AD
Reductant Level Sensor C Circuit
A general electrical fault is reported on DEF level sensor C — open, shorted, or out of plausibility. Some tanks use multiple level senders to cover irregular tank shapes; this is the primary one.
- 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.
- Sender failedCommon
Parts involved
- Reductant Level Sensor
P21AD 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.