OBD-II code · Powertrain (generic, extended)
P21A9 Reductant Level Sensor B Circuit Range/Performance
DEF level sensor B reading is in electrical range but plausibility check fails — value sticks, jumps, or does not match consumption.
Is it safe to drive with P21A9?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyFloat arm sticks
- Less likelySensor element aged beyond calibration window
- Less likelySensor mounting loose - signal damping
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
- P203BReductant Level Sensor A Circuit Range/Performance
- P21AEReductant Level Sensor C Circuit Range/Performance
- P21A6Cylinder 11 Air-Fuel Ratio Imbalance
- P21A7Cylinder 12 Air-Fuel Ratio Imbalance
- P21A8Reductant Level Sensor B Circuit
- P21AAReductant Level Sensor B Circuit Low
- P21ABReductant Level Sensor B Circuit High
- P21ACReductant Level Sensor B Circuit Intermittent/Erratic
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.