Powertrain, generic (extended)

P21B0 Reductant Level Sensor C Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P21B0 means GM's level sensor 3 (sensor C) shows an open circuit or a short to battery: either 3.56 to 4.74 V or more than 4.74 V, measured 1.5 ms after a test pulse, for over 3 s. The 6.6L LML and the 2014 Cruze both rate it Type A, lighting the MIL on the first trip. PIP4887B names P21B0 next to P203D and P21AB on 2010–2011 Duramax trucks showing Service Exhaust Fluid System; when all three high codes appear together, GM directs technicians to wiring and terminal tension first and to the DEF tank assembly only afterwards. A tank connector left unplugged after other work would produce the same set of codes, so check that before ordering parts.

Likely causes

Tank connector loose, unplugged or with spread terminals
Sensor 3 signal wire open or shorted to voltage
Level sensor C open inside the tank assembly

Symptoms

  • Check engine light on at once
  • Service Exhaust Fluid System message
  • P203D and P21AB often stored as well

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–1,200
Time0.5–4 h

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

Parts involved

reductant level sensor

Related codes

P21ADP21AFP203DP21AB

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