P21AAReductant Level Sensor B Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21AA means the second reductant level sensor reads too low a voltage, which GM takes as a short to ground. GM documents it for the 6.6L Duramax LML and the 2014 Cruze 2.0 diesel: the tank level electronics apply a test pulse, and if sensor 2 then shows less than 0.17 V for more than 3 s with over 8 V supply, the ECM receives an error status over CAN. The LML treats it as Type A (MIL on the first trip), the Cruze as Type B (two trips). GM bulletin 12-06-04-002C lists P21AA among the DEF codes that must run and pass before the warning tests can clear the driver messages. Look for chafed or wet harness sections first; as the sensors belong to the tank assembly, a failed element usually means a new tank unit.
- What you may notice
- Check engine light on
- DEF warning message that stays on
- DEF range or level display wrong
- Likely causes
- LikelySensor 2 signal wire chafed through to ground
- PossibleFluid or water in the tank connector
- PossibleInternal short in level sensor B (tank assembly)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.