P21B1
Reductant Level Sensor C Circuit Intermittent/Erratic
Reductant Level Sensor C Circuit Intermittent/Erratic: the signal is intermittent or erratic. Often a loose connector or chafed wiring; a wiggle test often locates it.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
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.
- Corroded sender contactCommon
- Connector with intermittent contact under vibrationLess likely
- Harness routed against vibrating componentLess likely
Parts involved
- Reductant Level Sensor
P21B1 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.