Powertrain, generic (extended)

P21B1 Reductant Level Sensor C Circuit Intermittent/Erratic

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P21B1 marks level sensor C signals that come and go. Public manufacturer criteria do not exist; GM's diesel tables describe sensor 3 only through the steady faults P21AF and P21B0, each filtered over 3 s. A stored P21B1 therefore suggests interruptions too short for those monitors, typical of a connector that works loose under tank vibration, fluid seeping into the terminals, or a harness pinched under a tank strap. Moving the harness while watching the sensor data with a scan tool is the quickest way to reproduce it. If P21AC is present too, the cause probably lies in wiring both sensors share. Drivers mainly notice an unsteady DEF level display or a warning that appears and clears again.

Likely causes

Tank connector loosening under vibration
Fluid seeping into the terminals
Harness pinched under a tank strap

Symptoms

  • Unsteady DEF level display
  • DEF warning that appears and clears
  • Stored code without current fault

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–1,000
Time0.5–3 h

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

Parts involved

reductant level sensor

Related codes

P21ADP21AFP21B0P21ACP203E

References: law.cornell.edu, gsi.ext.gm.com, static.nhtsa.gov.