OBD-II code · Powertrain (generic, extended)
P21A8 Reductant Level Sensor B Circuit
A general electrical fault is reported on DEF level sensor B — open, shorted, or out of plausibility. Some tanks use multiple level senders to cover irregular tank shapes; this is the primary one.
Is it safe to drive with P21A8?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelySender failed
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)
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–4 h
- Emissions test
- Likely to fail
- Parts involved
- Reductant Level Sensor
Related codes
- P203AReductant Level Sensor A Circuit
- P21ADReductant Level Sensor C Circuit
- P21A5Cylinder 10 Air-Fuel Ratio Imbalance
- P21A6Cylinder 11 Air-Fuel Ratio Imbalance
- P21A7Cylinder 12 Air-Fuel Ratio Imbalance
- P21A9Reductant Level Sensor B Circuit Range/Performance
- P21AAReductant Level Sensor B Circuit Low
- P21ABReductant Level Sensor B Circuit High
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.