OBD-II code · Powertrain (generic, extended)
P21AA Reductant Level Sensor B Circuit Low
DEF level sensor B reads below lower plausibility — short to ground. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
Is it safe to drive with P21AA?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyShort to ground
- Less likelyConnector pins corroded - high contact resistance
- Less likelyHarness chafing through insulation
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
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.7–3 h
- Emissions test
- Likely to fail
- Parts involved
- Reductant Level Sensor
Related codes
- P203CReductant Level Sensor A Circuit Low
- P21AFReductant Level Sensor C Circuit Low
- P21A7Cylinder 12 Air-Fuel Ratio Imbalance
- P21A8Reductant Level Sensor B Circuit
- P21A9Reductant Level Sensor B Circuit Range/Performance
- P21ABReductant Level Sensor B Circuit High
- P21ACReductant Level Sensor B Circuit Intermittent/Erratic
- P21ADReductant Level Sensor C Circuit
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.