Powertrain, generic (extended)

P21AA Reductant Level Sensor B Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Sensor 2 signal wire chafed through to ground
Fluid or water in the tank connector
Internal short in level sensor B (tank assembly)

Symptoms

  • Check engine light on
  • DEF warning message that stays on
  • DEF range or level display wrong

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,200
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant level sensor

Related codes

P21A8P21ABP203CP21AF

References: gsi.ext.gm.com, gsi.ext.gm.com, static.nhtsa.gov.