Powertrain, generic (extended)

P21AF Reductant Level Sensor C Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21AF is set when level sensor C, called level sensor 3 by GM, reads below 0.17 V right after the tank electronics apply their test pulse, which GM interprets as a short to ground. Supply must be above 8 V and the error must persist for more than 3 s. The 2014 Cruze diesel stores it as Type B, so the MIL lights after the second failing trip; on the 6.6L LML Duramax it appears in the same form, and GM bulletin 12-06-04-002C places it in the first group of DEF codes to be repaired before the warning tests can run again. When P203C or P21AA are present as well, several sensors reading low at once point to a common wiring or connector problem rather than to sensor 3 alone.

Likely causes

Sensor 3 signal wire shorted to ground
Water or fluid bridging contacts in the tank connector
Level sensor C shorted inside the tank assembly

Symptoms

  • Check engine light after two trips
  • DEF warning message
  • Implausible DEF level shown

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

P21ADP21B0P203CP21AA

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