P21AE: Reductant Level Sensor C Circuit Range/Performance

What does P21AE mean?

P21AE stands for an implausible level sensor C signal whose voltage is still electrically valid. No manufacturer describes a separate check for this code in public material. GM shows how such plausibility testing can work: on the 2014 Cruze diesel the ECM compares the readings of all three discrete level sensors after the test pulse and flags combinations that cannot physically occur together, but files the result under P203B rather than under a code for the sensor concerned. For the 2018 Cruze, GM did assign P21A9 to sensor 2. A maker using P21AE would point to sensor C itself, for instance after urea crystal deposits or partial freezing. Electrical faults of this sensor are covered by P21AF and P21B0.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• DEF level display does not match the refill amount
• Possible DEF warning sequence

Common causes

• Medium Crystallised urea on level sensor C
• Medium Partly frozen fluid around the sensor
• Low Level sensor C drifting or degraded

Likelihood is a general estimate across vehicles; your make and model may differ.